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11-08-2016, 04:22 PM   #1
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IQ of FF vs APS-C primes on APS-C bodies

I'm curious to know what qualities if any make FF prime lenses desirable over the cheaper APS-C versions (such as Pentax DA) when specifically used on APS-C bodies.

For example I recently picked up a DA 50/1.8 for my K-50. My previous camera was a Nikon DX-body and I had a Nikkor 50/1.4G which was a FF lens. The Nikkor 50 was considerably larger/bulkier and more expensive than the DA 50. It was also a little faster but not sure if that explains the whole price difference. It was still plastic and also did not have an aperture ring or distance scale. (correction: it did have a scale... I just always ignored it)

Is the added bulk and cost simply due to being a FF design? Or should I expect the image quality from a crop-sensor lens to be intrinsically different in some way?

Thanks!


Last edited by lightbox; 11-08-2016 at 05:09 PM.
11-08-2016, 04:39 PM   #2
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That half stop of light adds a lot of weight, but usually it's also associated with higher performance.

The DA 50mm is pretty much an FF lens in terms of coverage, but it's not quite as good in the corners as a dedicated FF lens would be.

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11-08-2016, 04:42 PM   #3
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The way I see it is that an FF lens has an FF size image circle while most APS-C lenses have a smaller image circle. Since all lenses have weaker edge sharpness compared to centre sharpness, the APS-C cameras get to snap the best part of the FF image circle (the centre) of an FF lens which theoretically should be good for sharpness.
11-08-2016, 04:56 PM - 2 Likes   #4
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FF designs are bulkier and costlier. They require larger glass elements and often require more complex designs to correct aberrations out to the farthest corners. To a first approximation an FF lens has about 3X the weight in glass of the equivalent angle-of-view APS-C lens (and probably about 1.5 to 2X in other materials).

It's hard to say anything general about the IQ differences of APS-C and FF lenses on APS-C sensor due to the wide variations in prices, brands, and design ages. The one IQ area where FF lenses may be generally inferior to APS-C lenses on an APS-C camera is flare from out-of-frame light sources. If you have an FF lens on an APS-C camera, you may think the sun is out of the frame but it's not. In general, the FF lens is capturing an FF frame worth of light but the sensor is only seeing the APS-C frame of it. All that extra FF light is bouncing around inside the lens and mirror box and potentially flaring or fogging the image.

11-08-2016, 05:01 PM   #5
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At low Iso numbers, given a common MP number for APSC and FF , given a FF lens put on different bodies (APSC and FF), you'll notice not much of a difference except for distortion and bokeh due to the crop factor (in fact there's no difference, you'll see a difference because with the APSC body you take just a smaller part of the image circle of the FF lens, presumed the better, and so cropping the original FF image and changing the relations between subject and frame. That's for low ISO.
If you talk about native APSC lenses (DX means cropped format with Nikon, if I'm right) yes. it could be that they're optimised for cropped sensor thus giving better IQ depending on which lens you refer and which ones you compare. Best regards.Matteo
11-08-2016, 05:13 PM   #6
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There is one side benefit to a FF lens...if leaves your options open when you buy your next body.
11-08-2016, 05:15 PM   #7
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QuoteOriginally posted by photoptimist Quote
To a first approximation an FF lens has about 3X the weight in glass of the equivalent angle-of-view APS-C lens (and probably about 1.5 to 2X in other materials).

It's more complicated than that, Photoptimist.


As a general principle, I agree more glass should be involved, but it depends on the compromises the designers are willing to accept.


Do you own many legacy lenses?


Have a look at the K 55 f1.8, or FA 35 f2 or FA 77 f1.8 or whatever.


They don't contain triple the glass of a DA50, DA35 or DA70, they'd be almost the same.


A lens like the M20 f2.8 *should* be much bigger, however, so of course it can't compete IQ wise with some highly corrected Samyang or Sigma prime.
11-08-2016, 05:20 PM   #8
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Talking about pure sharpness, I remember an interesting article referred to legacy medium format lenses used on cropped sensor. The sum is this: legacy MF lenses are optimised for a frame much more bigger than APSC , thus their resolving power is different and inadequate to high MP cropped sensors. So, speaking of pure sharpness, it's a non- sense using MF lenses via adapters on APSC. I'll post the link (it was in Italian language). That's for sure an extreme case and sharpness is not just the only thing to consider in a lens.

11-08-2016, 05:58 PM   #9
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On an APS-C body, i would definitely opt for DA primes (15, 21, 35 LTD, 40, 70).
11-08-2016, 06:12 PM   #10
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QuoteOriginally posted by clackers Quote
It's more complicated than that, Photoptimist.


As a general principle, I agree more glass should be involved, but it depends on the compromises the designers are willing to accept.


Do you own many legacy lenses?


Have a look at the K 55 f1.8, or FA 35 f2 or FA 77 f1.8 or whatever.


They don't contain triple the glass of a DA50, DA35 or DA70, they'd be almost the same.


A lens like the M20 f2.8 *should* be much bigger, however, so of course it can't compete IQ wise with some highly corrected Samyang or Sigma prime.
Yes, "it's complicated." Yet the list of DA lenses that work on FF would seem to suggest those DA lenses have an FF image circle so it's not surprising they are about the same size.

And comparing an old M lens to a highly corrected modern lens also goes well beyond the notion of "first approximation" which assumes we are comparing lenses of similar vintage, price point, and design goals with the exception of one lens being optimized for narrow image circle (APS-C) and the other optimized for a much larger image circle (FF).
11-08-2016, 08:33 PM   #11
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Some interesting points. I never considered the possibility of more flare in a FF lens on APS-C, for example.

Please continue the technical conversation (it's enlightening) but I'll now also pose a slightly different question: If we are not pixel peeping for sharpness, is there any artistic or compositional reason to choose one type of lens over the other on an APS-C camera?

Asked differently, if I were to take two identical portraits- one with a 50mm crop lens, and one with a 50mm FF lens- both on an APS-C camera and both at an equal f/4 (say), then what visual differences could I expect? Again, putting the resolution and sharpness aside for a moment.

(I think the answer is "none" to both questions, but I'm just guessing)
11-08-2016, 09:27 PM   #12
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QuoteOriginally posted by photoptimist Quote
FF designs are bulkier and costlier. They require larger glass elements and often require more complex designs to correct aberrations out to the farthest corners. To a first approximation an FF lens has about 3X the weight in glass of the equivalent angle-of-view APS-C lens (and probably about 1.5 to 2X in other materials).
When FF and APS-C lenses share the same performance level, meaning they can shoot the same FOV and DOF then they are pretty much the same weight and cost to build. With smaller format lenses they have to be built to a tighter tolerances to preform at the same resolution level and this adds cost to the lens just look no further than 4/3 some of the best optical made primes while a K1 and any DFA zoom will out perform them in a heartbeat. The larger the format the more loosy goosy they can design the lens and you have to look no further than 6x7
11-08-2016, 09:39 PM   #13
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QuoteOriginally posted by lightbox Quote
Some interesting points. I never considered the possibility of more flare in a FF lens on APS-C, for example.

Please continue the technical conversation (it's enlightening) but I'll now also pose a slightly different question: If we are not pixel peeping for sharpness, is there any artistic or compositional reason to choose one type of lens over the other on an APS-C camera?

Asked differently, if I were to take two identical portraits- one with a 50mm crop lens, and one with a 50mm FF lens- both on an APS-C camera and both at an equal f/4 (say), then what visual differences could I expect? Again, putting the resolution and sharpness aside for a moment.

(I think the answer is "none" to both questions, but I'm just guessing)
I'll take a stab at this, but if you really want to have fun just look for an article from a micro four thirds fan.

The larger the sensor the wider a lens field of view becomes since more of the glass is used to cover the sensor. A 50mm on ff is truly 50mm just as it would be on a APSC, however the APSC field of view is 1.5 times narrower since only the center most part of the lens is used. Also, the blurr for wide aperture is slightly less the smaller the sensor.

So apples to apples, the images would have a different field of view and blurr would be minimized a little.
11-09-2016, 12:21 AM - 2 Likes   #14
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Long before I got a K-1, I compared the DA40 with the FA43, and the DA70 with the FA77 on my APS-C bodies. I kept the two full frame lenses because they have wider maximum apertures without being too much bigger and heavier, and they have a different character to their rendering.

Call it pixie dust if you like, but a combination of LoCA, fringing, field curvature, coma or other FA limited "faults" gives their images the edge over the more corrected and (dare I say it) clinical DA's. Theoretically, a full frame lens should have sharper corners on APS-C than a crop lens, but the DA40 and DA70 were very good in that regard anyway, so that wasn't an issue for me.

Conversely, there's no chance I will ever use my FA20 or DFA15-30 @ 15mm on a crop body in favour of my DA21 or DA15.
11-09-2016, 12:32 AM   #15
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Sorry... editing my previous post I deleted it.... had a good cup of coffee and I'm back .

To answer the OP question: yes, some differences from lens to lens could be visible , just for the fact that lenses are different in design and the sensor is different. And f/ numbers and t/ numbers changes from lens to lens, so it's difficoult to use the exact same parameters with both lenses without any compensation depending on different light transmission.

For visual purposes I would compare PRINTED results cross-format, starting from the FOW point (of view ) . given the crop factor of 1,5 , an APSC lens like, i.e. 70mm 2,4 on APSC would give an eq. FOV of a 105 mm (f/3,5 ? does it exist?) on FF. That could be a fair (with some drawbacks) comparison, using 70mm on APSC boby and 105 on FF. Results have to be taken cum grano salis due to the intrinsic differences across the formats and the differences in lens' design and rendering.

The summary is that, in my opinion, APSC system isn't restrictive , if you already know the visual results you are aiming. Just choose the right lenses and work with those. Best regards. Matteo.

Last edited by bm75; 11-09-2016 at 01:30 AM.
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