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Old Dec 8, 2010, 4:00 PM   #1
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Default Full Frame lenses on APS-C

Do some of the full frame lenses that we have access to in great abundance not make the transition to APS-C well?

I started this because of something on another thread and something I think I recently noticed. The comment made on the other thread was a dislike for 85mm f/1.4 lenses on APS-C because the angle change made the lens less than optimum at its prime focal length.

In my personal case it is a Minolta Maxxum on the Sony A550. I did some shots of one of the Nieces at her recent pageant. It seems to my eye that the shots from the 50mm f/1.7 were much clearer than from the 85 and somehow that does not seem correct.

Are there any thoughts on this or is this something well known and I just missed the memo?

Steve

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Old Dec 8, 2010, 5:17 PM   #2
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When you use a 'Full Frame' lens on an APS-C dSLR, you're using just the center portion of the image circle projected by the lens. That excludes the portion of the image circle where the edge and corner sharpness isn't very good and where the vignetting occurs. So, in general, you will get improved image quality from a lens on an APS-C dSLR than you'd get with the same lens on a 'Full Frame' dSLR.

There is one exception, however: Digital image sensors are much more reflective than film, so light will often be reflected off the image sensor back toward the rear of the lens. Newer lenses, made for digital dSLRs have additional coatings on the rearmost optical element, to prevent this reflected light from being reflected back at the image sensor which will cause flare.

But in your case, your Minolta 50mm f/1.7 and your 85mm f/1.4 are both 'Full Frame' lenses, so you're using the sweet spot on both of them. But it is possible that your 50 has the digital coatings and your 85 doesn't. That may account for the difference you're seeing.
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Old Dec 8, 2010, 8:53 PM   #3
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...That's the portion of the image circle where the edge and corner sharpness isn't very good and where the vignetting occurs. ...
is this really what you meant to say?
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Old Dec 9, 2010, 5:59 AM   #4
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is this really what you meant to say?
No. You're right. I meant the reverse. I corrected my original post. Thanks for pointing that out.
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Old Dec 9, 2010, 6:13 AM   #5
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while i agree for the most part - i think it depends on the quality of the lens - if a full frame lens can only support say a resolution of 18MP before it is limiting the image not the camera sensor and then you place that lens onto an 18MP aps-c camera then the image being recorded in the aps-c image circle will be over the resolving power of the center section of full frame image circle of the lens

i dont think this is a huge problem yet but with megapixels getting so high these days some lenses are showing they were not manufactured with such considerations in mind

however if a lens is considered good on full frame chances are its even better on aps-c for the reasons tcav mentioned - just that if its a older lens then might be some issues
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Old Dec 9, 2010, 10:08 AM   #6
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These are both pre-digital lenses. After reading the sticky on this page and a few past discussions of the subject I think I have a good grasp of crop factor (the 16mm Minolta fisheye demonstrates this well as on my Sony I have an extra wide lens without the most extreme of the curvature). Those discussions have, in part, raised this question.

Angle of View changes with crop factor but not the actual focal length of the lens. With the narrower Angle of View we now have to be further away from our subject to obtain the same framing. In the case of the specialty lenses does this added distance create an issue?

My books are packed waiting for a move so I cannot get to it but I remember the book for the 85 f/1.4 making mention of optimum distances from your subjects. (the lens and book date to the late 80's)

Thank you for the input

Steve
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Old Dec 9, 2010, 10:45 AM   #7
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You have to take each lens on a case by case basis

Some may be test sharper than others with the aperture wide open, while testing great down a stop, etc. Same thing for corner sharpness as you get further away from the center.

Some are going to do better than others (and in some cases, you may not care if the corners are soft with the aperture wide open, because that area is usually out of focus anyway.

Some of the MTF charts can be interesting when shopping for lenses, since lenses that may not have graded as well overall, still do well on a camera with an APS-C Size sensor because of the way sharpness and contrast don't drop off in some cases until you're further away from center than an APS-C sensor can cover.

For example, here are some MTF charts for the Minolta 85mm f/1.4G, where I used waybackmachine.org to go back to an earlier version of photodo.com, since it's much harder to navigate to MTF data and charts with the new site layout (the charts are under a different images option on the new site).

http://replay.waybackmachine.org/200...0_14-387.shtml

Note how you've got a dropoff in sharpness at around 15mm from the center in some of the measurements at f/8 with the 85mm f/1.4G. So, you might have slightly softer corners with a full frame camera. But, if you use that lens on a camera with an APS-C sensor, you wouldn't care, because it's dropoff in sharpness wouldn't occur until you started to leave the image circle size used by the smaller APS-C size sensor.

At f/1.4, the rolloff starts sooner but is also more gradual (versus a flat line that drops off more sharply) as you move away from center, and with it wide open at f/1.4, you probably wouldn't care much about the corners anyway, as they're probably going to be out of focus for the types of photos you'd be taking with the aperture set that way. They don't give the graphs for in between aperture settings (and most lenses sharpen up considerably as you start stopping them down some). But, the graphs they show and the MTF numbers for other apertures should give you a general idea of typical behavior on full frame versus APS-C.

Now, as already mentioned earlier, a higher resolution sensor can place more demands on the lens quality needed for best results, and in the case of the latest APS-C size sensors, the pixel density is higher, requiring a lens to be able to resolve more detail to take full advantage of the extra resolution.

In other words, because the pixels are packed much closer together on an APS-C size sensor for a given resolution in megapixels, the lens resolving power needs to be greater to take full advantage of the sensor's resolution.

For example, the original Konica Minolta 18-70mm kit lens (carried over with a Sony brand name later), was "good enough" for a 6MP sensor.

But, when higher resolution models came along, there became a point when the 18-70mm lens simply couldn't resolve well enough so that the higher resolution sensor offered any benefit with it (as in comparing images from a model like the 10MP Sony A300 versus the 14MP Sony A350 when wearing that lens), due to higher pixel density.

So, if you wanted better results with the higher density sensor, you needed a better lens than the 18-70mm kit lens Sony was shipping with it (and Sony did redesign the kit lenses used on later cameras, switching to a newer 18-55mm design). Here's a review page showing a good example of that problem:

http://www.cameralabs.com/reviews/So..._results.shtml

You'll see more discussion in this post, where I pointed out some tests of Nikkor lenses on a D3 versus D300 (where both are 12MP, but the lenses resolved more detail on the D3, testing much better on charts (at least towards the center of the frame). The D3's larger sensor has lower pixel density (fewer pixels/mm) compared to the APS-C size sensor in the D300, allowing for larger photosites with more surface area. So, that places less demands on the resolving power the lens needs to get the best results.

http://forums.steves-digicams.com/wh...ml#post1161469

But, you have to take each lens on a case by case basis for the apertures you'll be shooting at.

For example, when I bought my Minolta 100mm f/2, I also considered the Minolta 85mm f/1.4 (as I could have bought either one for about the same price at the time), and settled on the 100mm f/2, since I knew it would be used wide open at f/2 most of the time, and it tests a little bit better at f/2 compared to the Minolta 85mm f/1.4G stopped down to f/2. Of course, you don't get f/1.4 with the 100mm lens. lol

You'll find the old pages here (just click on Minolta, then click on a lens to see MTF test data and charts on the same page. Note that after you click on a specific lens to see charts, close the window that opens before clicking on a different lens in order to see charts for another one.

http://replay.waybackmachine.org/200...prodindex.html

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Old Dec 9, 2010, 10:52 AM   #8
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Of course with specialty lenses the angle of view can become an issue. First and foremost you can't always "move back" - so any time you're talking wide angle lenses that can be an issue. I think it's a little less of an issue when you are talking portrait lenses - but you can still extrapolate - if you're taking portrait style shots in your living room you can only back up so much before you hit a wall but you generally have more room if you're doing half-body or head shots you were probably pretty close and have room to move back a bit. So the impact isn't as great as it is on the wide angle.

Now, one of the other points is, in general, the standards for 'good' have gone way up. Because we take a lot more digital shots and we look at these on computer screens, I think it's easier to spot flaws in old film-based consumer grade lenses. We were happy when using film but suddenly we see the flaws much more easily with digital. Not the least of which because we now see a lot more photos from other photographers using better quality lenses. So I think the bar for "good" is a lot higher now than it was 20 years ago. Now some of the best film era lenses still perform great. Just my opinion.
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Old Dec 9, 2010, 12:05 PM   #9
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Originally Posted by Old Boat Guy View Post
...I did some shots of one of the Nieces at her recent pageant. It seems to my eye that the shots from the 50mm f/1.7 were much clearer than from the 85 and somehow that does not seem correct.
That could be for lots of reasons, other than anything to do with lens quality, too (and you'd probably want to stop the 85mm f/1.4 down to f/2 for that type of shooting, as it is a bit softer at f/1.4). As a general rule of thumb, unless you're shooting a specific subject where softness away from center is desirable, it's a good idea to stop down most f/1.4 lenses a bit from wide open for better results. I'd use it at f/2 or so for something like a pageant.

Lens performance at a given aperture aside, if you're shooting from the same distance with both a 50mm and 85mm lens, any blur from subject movement would be more obvious on the shots from the 85mm lens. That's because your subject would occupy a greater percentage of the frame, and any blur would be occurring across a greater distance in the final image for a given viewing/print size. So, you can sometimes get away with a slower shutter speed if your subjects represent a smaller portion of the frame before blur from subject movement is noticeable for a given viewing/print size.

That's one reason why sharpness can look OK with a downsized image, when you may see some blur from subject movement and/or camera shake when viewing the image at larger sizes.

If the movement is not occurring across a great enough distance in the final viewing size, it may not be noticeable to the human eye. So, the wider angle of view can seem sharper in some cases, even though you're really capturing less detail in many cases (because of less pixels representing your subject), if you tried to enlarge the subject to match what you got with the longer focal length lens.

There are many variables involved. ;-)
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Old Dec 9, 2010, 12:38 PM   #10
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...So, you can sometimes get away with a slower shutter speed if your subjects represent a smaller portion of the frame before blur from subject movement is noticeable for a given viewing/print size.
More on that subject...

Speed of movement, direction of movement and more come into the equation when determining what looks sharp to the human eye.

This shutter speed calculator is no longer online, but it's still available via waybackmachine.org. If you scroll down and look at the section describing "Print Blur", you'll see the calculations are based on the human eye seeing movement as being frozen if it's 0.5mm or less at an 8x10" print size.

http://replay.waybackmachine.org/200...om/spcalc.html

Many of the DOF calculators, sharpness calculators for shutter speed needed, etc., are based on a given viewing/print size, using typical viewing distances for those sizes to determine what the human eye detects as being sharp.

In the case of blur from subject movement, if your subject occupies a smaller percentage of the frame (as you'd have using a shorter focal length lens from the same distance when shooting a moving person), it may seem sharper for a given viewing/print size because blur is less obvious, because movement occurring across a shorter distance at the size being viewed (with around 0.5mm or less movement appearing to be frozen to the human eye, at typical viewing distances using an 8x10" print size).

If you're shooting a pageant in low light, you may have a bit of blur from subject movement from slower shutter speeds that's more noticeable using the longer focal length lens from the same shooting distance if shutter speed is the same, even though you're really capturing less detail with the 50mm at a given aperture setting because you have less pixels representing your subjects (provided you're not using the 85mm wide open, with similar settings for aperture and shutter speed with both your 50mm and 85mm).
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