Lens Review Methods and Terminology - Photo Jottings

Lens Review Methods and Terminology

Below is an explanation of my review methods and terminology.  Hopefully this will help clear up any questions you may have concerning how I test a lens. 
I set up this page the same as a regular lens review, so if you want testing information about something in a certain part of a lens review, go to the same section below.
SAL-??F28  Sony ??mm F/2.8  This line lists any product code and/or official retail name for the lens.
Box contents
Listed here is what came in the box, such as a case, hood and owners manual.
I list Manufacturers retail price for Sony lenses.  Street prices are for aftermarket lenses like Sigma which often sell for a deep discount rather than their MSRP.  Used lenses like Minolta are sometimes listed using current eBay prices.
Build quality
Build quality means fit and finish, plus mechanical performance like zoom dampening, and focus control.  The lens doesn’t have to be built heavy with all metal construction to be labeled “very good.”
Additional information
Included here would be information such as “the same as tamron model xx” or “rebadged Minolta” which means in both cases the lens is being used by Sony, though they didn’t design it.  Also, additional info could list lower cost alternatives to the model in review, or any out of the ordinary feature of the lens.
Specifications below
Optical configuration
XX elements in XX groups.  Self explanatory, and not really useful for the photographer.
Angle of view
This is the coverage of the lens, usually measured on the diagonal.  There are different coverages for full frame and APS-C, that’s why I list them separately.  The larger the number, the more area you can see through the viewfinder.  Angle of view numbers tend to be less than accurate for some reason, so don’t try to compare them directly to other similar lenses.  I use the factory specs when available (most of the time), and guess when unavailable (rare).
How many blades make up the aperture.  Usually 7 or 9 for Sony, and 6-8 for Sigma.  Sony uses mostly curved blades (circular aperture) now, as opposed to straight blades.  Out of focus areas may look better with curved blades at wide open to two stops down, straight blades may show a polygon shape at that aperture.  When stopped down hard there is no difference between the two.
Full frame and APS-C
All Sony/Minolta/Konica Minolta AF lenses will work on any Sony/Minolta/Konica Minolta AF camera.  This includes compatible after-market lenses also.  If you use a film or full frame digital camera, you may not be able to use the entire area with “DT” lenses, which have a smaller imaging circle, and are built for APS-C cameras only.  If you use a made for full frame camera lens on an APS-C camera, you’ll have to figure in a crop factor of approximately 1.5 (Sony) when figuring coverage.  So a 50mm lens used on an APS-C camera would produce an image with coverage equal to 75mm, 50 X 1.5 = 75mm.
I also list full frame coverages of “DT” lenses which are for APS-C cameras only.  I do this because some of those lenses show almost full coverage when mounted on a full frame camera, so if Sony offers an option in the future which allows use of “DT” lenses without the automatic crop feature, you may be able to cut down on the lenses you carry or have to buy.  It’s not likely to make much difference, but I show it for those people that are curious.
Depth of field and focus scales?
Depth of field scales are usually seen on non-zoom lenses like 50mm F/1.4, 28mm F/2.8 etc, but occasionally come on zoom lenses, like the Sigma 12-24mm.  Focus scales are common, and show up either written on the barrel or in a window.  Cheap lenses usually show nothing.
Minimum focus, image plane to subject
This is the distance the manufacturer gives for the closest focusing distance from the image plane to subject.  The image plane is the film or sensor.  Most of the time you can get closer than these figures.
Minimum focus, end of lens barrel to subject
This is not the same as above, and differs in that the closest focusing distance is measured from the end of the lens barrel, or farthest protruding part.  This is more useful to the average photographer.
Hard stop at infinity focus?
Some lenses cannot be focused past the ∞ mark (infinity), and stop at that point.  Some lenses will focus a short distance past the infinity mark, in part to allow for thermal expansion, and utilize the extra space to achieve proper focus.
Length changes when focusing?
Some lenses focus by extending the front section, usually not more than .5″ (6mm).
Focus ring turns in AF?
When in AF mode, and the AF is actually functioning, the focus ring will turn so you can’t hold that portion of the lens during this time.
Filter size
This is the size of filter that screws on to the front threads, and also is usually the same as the front protective cap, measured in millimeters.
Filter ring rotates?
If the filter ring turns, your polarizer and graduated filter will turn, messing up the alignment.  Now days, only cheap kit-type lenses have filter rings that turn.
Distance encoder?
Distance encoders (integration) are used primarily for proper flash output, and combine readings of the lens focal length, focusing distance and exposure reading .  If your lens has eight pins or square metal contacts on the back, it most likely has distance integration.  Not all Sony lenses have this.  If the lens has only five contacts, it doesn’t have it.
Max magnification
This is the maximum reproduction ratio, or magnification.  This figure will represent the equivalent coverage of 135mm film.  Look for different listing styles, but they mean the same, like 0.3x, which is about a third life size, also written as 1:3.  Also shown on some specs pages (for example), 1:4, which is the same as 0.25x, (quarter life size), 1:1 for 1.0x (full life size), or 1:7.1, the same as 0.14x.  Check this by dividing one by the following number to convert, like this; 1÷7.1=0.14. Or to convert the other way; 1÷.14=7.1.  The closer the number to 1.0, the larger the subject will be magnified.
Min. F/stop
The minimum (smallest aperture at that zoom length) is listed first at the wide part of the zoom, and the last is the telephoto end, such as F/22-F/29 etc.  Example; a Sony 24-105mm lens will have a minimum aperture of F/22 at 24mm, and F/29 at 105mm.  On a non-zoom, there’s only one minimum.  Also, the minimum F/number may be different if you use exposure steps in 1/2 EV, or 1/3 EV.  The default setting from Sony is 1/3 EV, and that’s the setting I use to list the minimum aperture numbers.  If the minimum aperture reads F/29 on your camera, it may read F/27 using 1/2 EV, that’s why there’re some discrepancies in these numbers from the Sony site specs, and my specs.  It’s no big deal, nobody uses those small apertures anyways, diffraction softens up the image immensely.
Sony teleconverter compatible?
Is this lens compatible with Sony tele-converters?  Any lens will take an after-market tele-converter, but I wouldn’t buy one at any price.  purchase only Sony tele-converters for Sony lenses for the best results.
Dimensions W x L (my measurements)
Width is the widest point on the barrel, though sometimes I give extra measurements from the filter ring, or a switch, which sometimes is the widest part.  The length is from the lens mounting surface to the end of the barrel at the shortest length, zoom fully retracted and focusing fully drawn in.
Maximum  extended length (my measurements)
Maximum length after zooming and any focus extension.  Measurements are from the lens mounting surface to the end of the barrel.
Weight bare (my scale)
I have my own calibrated scale, and use that for weighing the lens bare, and with caps and sometimes accessories, like tripod collars etc.  This may be different from the factory specifications.  Don’t trust Sony’s figures, they are sometimes way off.
Requisite product shots.
Bulging front element
Box contents with no box
X-ray view, Sony screen grab.
The product shots above (just a couple of random samples ) are used to identify the lens reviewed, with views showing the back, side, zoom extended, interesting features, box contents, front element and X-ray and/or MTF data.  All shots are of the actual lens reviewed, taken by me.  MTF data, when available, is usually incomplete, this is all I can find from the manufacturer, so I offer it to those that like to check out the graph for kicks.
In the section below the product shots, I provide information on cameras used for testing, build quality, focusing, interesting qualities or features, and all around performance.
Filter size.  As I said above, the size of the front threads, and the lens front cover size.
Normal filters.  Do the use of filters on the front cause any problem like vignetting, also called dark corners or light fall-off.  Most of the time I indicate the issue with regular filters, not the thin type which are more expensive, but cause fewer problems.
Coma.  Coma is an artifact of spherical aberration, and shows itself as oblong shapes mostly in the corners of images at wide apertures in wide angle lenses.  I usually provide sample crops.
Color.  How does the color look (on an image) as compared to other lenses from Sony, Minolta or Sigma?  I’m not talking about the actual color of the lenses or coatings, rather, the color produced on an image by that lens.  This is mostly hard to define, as the camera may process consecutive shots just a little differently, and is only noticeable as you flip through images on your computer.  I’ve found most Sony/Minolta lenses product a very similar color, and I’m not likely to mention any difference unless it’s very noticeable.
Bokeh.  How does the background highlight blur look?  Do bright points of light look like donuts or ring washers etc, or do they have the same smooth consistency from edge to edge?  I usually provide sample crops.  Background and foreground blur can look different, but background blur is much more common in general photography.  I may comment on foreground blur if it looks substantially different than background blur.
Color fringing.   Are there colors like red/green/purple or blue that occur along the image edges in areas of harsh contrast?  This is called lateral color fringing, and will not go away by stopping down the aperture.  Less common color fringing is called axial, and it can occur anywhere in the image, and is identified by a single color all the way around an in-focus object.  You may notice it in the central area, however, this type of color fringing is normally only visible at large apertures, and it goes away as you close the aperture a stop or two.
Random shots below.  Here I use sample crops and small, re-sized complete images to show how the lens performs optically.
Sun centered, F/5.6
october08/20mmsunctr.jpg october08/20mmffsunretro.jpg
Bokeh, F/2.8
Bokeh, F/4
october08/20mmbok28.jpg october08/20mmbok40.jpg
Coma general scene
Coma scene normal shot
Jan2009/comalmx.jpg Jan2009/cmaover2.jpg
Coma, F/2.8
Coma, F/4
october08/20mmcma28.jpg october08/20mmcma40.jpg


In the top row I usually show re-sized full images of light fall-off, and how things look with the sun in the picture.  Sometimes certain lenses will show a great deal of ghosts, also called color blobs, when the sun or other very bright light or reflection is close to, or inside the image.  The ghosts can become more, or less noticeable depending on the angle of the light source inside your shot.  I often show another example of how the image looks when the sun is centered.  This is usually only a problem for wide angle lenses as some of them show a large ring or “wagon wheel” around the sun when the sun is centered.  This may cause problems with rising/setting sun type shots.  Flare problems are shown here too, alongside ghosting.  Sometimes I use the term “veiling glare” which is a type of flare that may cover the whole image, giving it a soft, low contrast look.  Keep the following in mind when viewing the shots above; the severity of flare and ghosting as it appears on your image depends on many factors, including exposure level, angle of sun or strong light source, aperture, focal length and background content.


The last item to check in the top row or two would be light fall-off, vignetting or corner shading, all the same thing, use whichever term you prefer.  Here I occasionally show how the lens handles this in real pictures, as opposed to the light fall-off samples farther down the page.  I show the worst case, usually at the wide end, with the aperture fully open.


The middle crops show the image bokeh.  Here I take sample crops from out of focus areas, usually the central part of the image not far from behind the subject.  Typically, the points of light should be fairly smooth and even looking, not like donuts or ring washers as I said above.  The look of bokeh depends on many factors, including, background content, lighting intensity, distance from in-focus subject, aperture and of course, the lens itself.  Bokeh can have a different look between foreground blur and background blur.  Background blur is much more common in general photography, that’s why I show the background blur crops.  I use crops from the widest aperture, and one or two stops down.  If it’s a zoom lens, I provide samples from the widest and longest setting.  Keep in mind the crops are displayed at full size, and wouldn’t be very noticeable, if at all, in a normal sized print.


The last row, (or rows) show the signs of coma.  These small crops come from the corners of the image.  Coma turns points of light at the corners into weird blobs, sometimes resembling bats, airplanes or birds.  Most of the time you see this with normal to wide angle lenses with a large aperture, like F/1.4 or F/2 etc, but sometimes it’s bad on a super-wide lens with a F/2.8 aperture.  When stopped down to F/4, the problems diminish greatly.  Aspherical elements help solve this issue, but as expected, the worst coma problems comes from lenses which have no aspherics, such as the following that I’ve reviewed; Minolta 28mm F/2, Sony/Minolta 50mm F/1.4, and the Sony/Minolta 20mm F/2.8.  Don’t worry about this if you don’t take pictures in low light, with sharp points of light around the image periphery.  I don’t show coma crops if there are no problems with coma.


I use clear miniature lights on a fake tree in the corner of the room for determining the severity of coma.  I show the worst of coma, with the aperture wide open, and one or two other crops at smaller apertures.


Distortion below.


Barrel distortion.
Pincushion distortion.
No distortion.


Distortion sample shots come from a house window.  The top image makes the window look like it is bulging out, and is called barrel distortion.  The middle image seems to suck the window in, and is called pincushion distortion.  The last image is flat, and lacks any distortion, this is ideally what you want in a lens, but most often don’t get.


Light fall-off.


october08/20mmvig28.jpg october08/20mmvig40.jpg
Light fall-off,  vignetting or dark corners all mean the same thing.  I usually take pictures of a white acrylic panel with a light in the back.  I try to show the worst case, which comes with a wide open aperture at either the widest angle or fully zoomed in.  Stopping down the aperture helps greatly to reduce light fall-off.  These are full re-sized images, and haven’t been cropped.


This is a real life shot of light fall-off, and doesn’t seem to be so bad as compared to the test shots in grey.  I always shoot this image at the setting that shows the most light fall-off, again, at the maximum aperture, wide angle or full zoom.


Sharpness crops.




Here is the general location of the corner corps in the full image.


Jan2009/cornersmlcp2.jpg Jan2009/cornersmlcp.jpg


These crops are from the red box in the bottom left corner of the image above, I don’t normally show the whole image.  I start off at the widest aperture, then stop down in one stop increments up to where things don’t get any sharper, usually around F/8-11.  On zoom lenses I show crops from the widest setting, a middle setting, and the longest setting.  I list the focal length above the boxes.  These shots are almost always taken at infinity focus, and always re-shot at least twice to confirm accuracy.  Manual focusing is used and bracketed to make sure the shots are as sharp as they can be.


The centers next.




Here is the general location of the center corps in the full image.


Jan2009/ctrcpzblr.jpg Jan2009/ctrcpz.jpg


These crops are from the red box in the center of the image above, I don’t normally show the whole image.  The same procedures are used for the center crops as the corner crops.  I use manual focus, and bracket the shots until I get the absolute sharpest shots for each focal range, and aperture, then shoot the set.  Also, I use infinity test distances on most lenses, (except some telephoto) so there are no DOF issues to worry about.


I now put the sharpness crops at the end of the review because of my table width issues.



Close focus sample.
The standard text for this section follows, and is self explanatory.  Below, click for larger image.  Check out the close focus shot, a 100% cropped portion of the full image.  The sample shot was taken with the Sony A 700 12.2mp camera.  The subject is a standard US stamp, 1″x 3/4″ or 25.4mm x 19mm.  Also, note the macro shot was taken as close to the subject as focusing allowed; In this case, x.x” or xxxmm, measured from the front of the lens barrel to the subject.


This is typically the whole scene, or approximate capture area.  I crop the stamp portion and display it at 100%, most of the time with a link to a larger image.  I do this to save on bandwidth, and avoid excessive download times for people with dial-up service.  If the cropped size ends up being small, I don’t use a link to a larger image, the image on the page is as big as it gets.


Since this page was written, I’ve changed to a different stamp, but they’re both about the same size with the same detail.


Close focus shot.
My procedures for the stamp test shot at close focus are as follows; a tripod is used, then I start off at the maximum aperture of that lens, and work my way down to about F/11-16.  I use three testing methods for getting the sharpest shot.  The first is manual bracket focusing, I do this to allow for mistakes in auto-focusing, and mistakes by simply looking through the viewfinder to see what looks the sharpest to my eye, which may not be the sharpest shot on a computer screen.  The second procedure is using the auto-focusing system, and setting the stamp to just within AF range and shooting as you normally would.  The third and final method is moving the stamp board ever-so-slightly back and forth with the focus ring turned all the way to close focus, and using manual focus.  Tiny incremental movements in distance can cause noticeable changes in image sharpness when the lens focusing system remains unchanged.
Getting the sharpest macro, or close focus type shot can be extremely hard, and may require considerable thought and set up, especially using large apertures.  The higher the magnification, the harder it is to achieve the best shot.  Lenses with poor close focus ability are not so finicky, and may produce the best shot by simply using the AF system.
Finally, I post the sharpest shot out of the bunch, then show what aperture was used for that image.  Most lenses will have the sharpest test shot around F/8, or about two stops down from the maximum aperture.  Close focus type shots can be far different from infinity focus when comparing sharpness to aperture.  Don’t worry too much about the results of this section, because most people don’t take test shots of stamps, or other super fine detailed objects.  I only include it to show people what they can expect if they would like to indulge in this kind of activity.
Full frame section using the Sony A900.
In this section I show the differences in results when using a full frame (or film) camera, as opposed to the cropped sensor or APS-C sensor (as I call it) camera above.  The full frame sensor is larger, and uses the entire lens’ imaging circle, instead of the middle portion like that of the APS-C sensor when using a full frame compatible lens.  The middle of the lens is the good part, the periphery of the lens is what messes up the incoming light, and causes problems with your pictures.
Most of the differences between the two are light fall-off, corner sharpness, distortion and coma.  Normally, you need to stop down one or two stops to get the same results as the APS-C camera, that’s just the way it is.  Good full frame lenses may show almost the same results as the APS-C crops.  Distortion is the only item not affected (or helped) by stopping down, but most of the time it’s easy to correct in photo imaging software.
My final thoughts.

This section concludes the review with thoughts about how the lens functions on both APS-C, and full frame cameras.  I also include some information on topics such as ultra wide-angle use, or why a person would buy a lens of this type, and any good alternatives etc.


My newer reviews have the test crops in this area, which is at the very bottom of the page.  I do this as the crops tables take up more width than the standard page, and it makes it easier to view the rest of the review without having to scroll across the page.


I hope this page helps you understand my testing methods and the terms I use to describe results.  Let me know if I can do better.

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