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02-09-2011, 09:15 AM   #1
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APS-C vs FF : myth or reality?

Well, from time to time, I stumble upon the following comments about APS-C vs FF:
- Lenses are bulkier/heavier/more expensive on FF.
- APS-C allows for more compact gear.

But it all rings false to my ear...
Let me explain... And prove me wrong if I say something stupid!

First of all, the camera... There is nothing preventing a FF camera to be the size of a K20, apart maybe for the SR mechanism. The mirror and pentaprism, well, you just have to look at some film camera to see that they fit in even smaller housings than a K20... I guess Canon and Nikon feel obliged to produce big, pro-looking bodies, and thus the idea "FF=big" stuck to our minds...

Now the lenses...
Let's start by looking at equivalency... For all practical purposes, for FoV and DoF (and to a certain extent, ISO) equivalency, a FF lens has 1.5x the focal length and about a 1-1/3 stops smaller aperture.
This means that a 50/1.8 APS-C lens is the equivalent of a 75/2.8 FF lens, be it in terms of FoV, DoF or light sensitivity (as the FF sensor has a good one stop advantage here).

OK, Macro lenses are a slightly different matter that I'll let aside for the moment, and this equivalency is slightly false when focusing near hyperfocal distance (like, splitting hair false). But let's say we're talking about average use here.

So, looking at truly equivalent lenses, you can see that they are indeed similarly sized/priced between the two formats (when they exist), or that they simply do not even exist in APS-C.

For instance, let's take a wide angle:
- Sigma 12-24/4.5-5.6, DG (FF), 87x100mm : 789€
- Sigma 8-16/4-5.6, DC (APS-C), 74,4x105,7mm: 719€

They are nearly the same size, weight and price, the FF version being slightly bigger, but it has in fact a faster "equivalent" aperture in term of DoF (it would be a 8-16/2.8-3.5 in the APS-C world, so imagine what this would do to the size/cost of the APS-C version!!!).

Now, take a DA 16-50/2.8 APS-C lens. Its FF equivalent would be a 24-75/4.5 (which Pentax never produced), so let's take the FA 24-90/3.5-4.5 instead (more reach, faster).
- DA 16-50/2.8 : 84x99mm, 565g (but has SDM and quick-shift)
- FA 24-90/3.5-4.5 : 72x75mm, 355g

The full-frame lens is even smaller here!

Another example?
Take the Pentax 50-135/2.8, against its FF equivalent the canon 70-200/4 (a slightly faster aperture than the 4.5 equivalent):
- Pentax 50-135/2.8: 76x136mm, 685g, 950€
- Canon 70-200/4: 76x172mm, 700g, 560€

Not so much differences, no? And I would gladly exchange the 3.6cm size advantage of the Pentax for the 40% price drop of the canon!

Even our good old 28-80/3.5-5.6 kit lenses (72x74mm for an AL version) gave the same "feeling" as a 18-55/2.2-3.5... The nearest APS-C offering is the slower Sigma 18-50/2.8-4.5, and with its 74x88mm is indeed bulkier...

So, apart from the body size, it seems to me that saying APS-C is more compact/lightweight/affordable is a fallacy...


Last edited by dlacouture; 02-09-2011 at 09:29 AM.
02-09-2011, 09:45 AM   #2
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As far as I have understood, an F stop is an F stop, regardless of the size of the film plane behind it. Yes, they have a different apparent DOF on different sized sensors, but an F4 lens is still slower than an F2.8 lens any day of the week. Am I wrong?
02-09-2011, 09:58 AM   #3
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For the cameras : note that film cameras don't have a rear screen and all the boards. A digital cam is about 2cm thicker than a film camera, due to that

A quick comparison I made with my Canon 5D (24x36) and my Pentax K100D (APS-C). Same lens (Helios 40, 85/1.5 @f/1.5), same reproduction ratio :




The second is the one on the K100D, and I had to reduce contrast. On the 5D, I had to enhance it, so there is a big difference in dynamic range. We can also see a loss in lens' caract (swirly bokeh), pespective modification (see the importance of the chair in the bokeh), and dof is shallower with the 5D (giving also more OOF blur).

I think the guys who say APS-C cams are lighter/more compact does not compare them at equivalent dof. But for me, dof control/shallow dof are important, if not, I would use a simple compact cam with everything in dof. That's Why I had to buy the 5D, as Pentax seems to be willing to remain on the "small" formats.
02-09-2011, 10:10 AM   #4
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The istD was originally designed for a full-frame sensor and in fact has room for one- this shows that it is possible to make a compact full-frame.

However, most full-frame users care about features over portability.

02-09-2011, 10:12 AM   #5
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CarbonR - I'd love to see the lens compared with the 5d against a K5 - the DR difference would be out the window.

It seems to me the only real, obvious difference between the two sensor types at this point is out-of-focus highlights. APS-C will always be at the disadvantage here, there is no point in arguing that. You could say the same about FF vs Medium Format.

I've read/had discussions where people point out that it's been said there would never be a sub $1K aps-c DSLR, or a sub $2K FF DSLR. With the former being halved (many priced in $500-600 range), and a sub $2K FF camera halving the current offerings as well, how many years will it take for the $10K medium format camera to be halved? It is all about volume, supply and demand, and eventually it'll get there. Sorry for the rambling, haha, not much of a point here except to say I think Pentax doesn't see a FF market making any money for them.
02-09-2011, 10:48 AM   #6
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Sorry, I forgot to say that this is not another "Pentax has to go FF" thread, but I just wanted to share some thoughts about the most common argument in the APS-C vs FF discussion (beside the obvious body price!)...

And I left tele lenses out intentionally as they don't really are an argument... With a same generation sensor, you usually have the same pixel density (the D700 being quite a strange thing in this), so if you want the extra reach in FF you can always crop the image... And again, an equivalent lens would be the same size/weight/price anyway...

On the other hand, I could not find any APS-C lens above 300mm... They are all FF, so I guess you would be better with a FF body to go with them...
02-09-2011, 10:52 AM   #7
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QuoteOriginally posted by Adam Quote
The istD was originally designed for a full-frame sensor and in fact has room for one- this shows that it is possible to make a compact full-frame.

However, most full-frame users care about features over portability.
Humm, I don't know, seeing the success of the 645D, I guess a rugged, compact, light and pixel-packed Pentax full-frame would find its place in many a backpack all over the world...
02-09-2011, 11:14 AM   #8
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QuoteOriginally posted by pxpaulx Quote
CarbonR - I'd love to see the lens compared with the 5d against a K5 - the DR difference would be out the window.
That's simple. Send me a K-5 But the 5D/K10D belong to the same generation. To compare with a K-5, you should compare it to a newer FF camera, like a 5D II or something else

02-09-2011, 11:43 AM   #9
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QuoteOriginally posted by pxpaulx Quote
As far as I have understood, an F stop is an F stop, regardless of the size of the film plane behind it. Yes, they have a different apparent DOF on different sized sensors, but an F4 lens is still slower than an F2.8 lens any day of the week. Am I wrong?
You are correct.
02-09-2011, 12:28 PM   #10
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QuoteOriginally posted by pxpaulx Quote
As far as I have understood, an F stop is an F stop, regardless of the size of the film plane behind it. Yes, they have a different apparent DOF on different sized sensors, but an F4 lens is still slower than an F2.8 lens any day of the week. Am I wrong?
with 1 stop difference you are correct but with 1/2 or 1/3 you may not be...
coatings and glass used play huge advantage in light transmission.
As example, my 35 years old K50/1.2 is not really 1 stop faster than my FA43ltd in terms of shutter speeds I'm getting. Numerically though the difference is just over one stop....

As for OP,
I do get what you are saying (partially). To gain the same FOV/DOF combination the APSC lenses are not smaller in any case I came across. And since FF sensors would easily fit into K-5 ish size body IMO, I think APSC will sooner or later get squeezed out by FF and m4/3

my2p
02-09-2011, 12:47 PM   #11
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QuoteOriginally posted by pxpaulx Quote
As far as I have understood, an F stop is an F stop, regardless of the size of the film plane behind it. Yes, they have a different apparent DOF on different sized sensors, but an F4 lens is still slower than an F2.8 lens any day of the week. Am I wrong?
You're of course right, but given that DoF is driven by the F stop, and that the easiest way to express DoF equivalence between two formats is by giving the equivalent F-stop needed to obtain the same DoF in the other format, we are forced to use such shortcuts...

Anyway, as full-frame cameras usually have easily a 1 or 2 ISO stops advantage over APS-C, I feel the conversion is quite right.
So, a 300/4.5 on full frame is indeed the true equivalent of a 200/2.8 on APS-C, on all levels:
- size,
- weight,
- price,
- noise level (as you have to use a 1-1/3 higher ISO level to have the same shutter speed)

I guess this is because we have the same registration distance for APS-C and Full-frame. With a smaller distance, surely more compact lenses could be made...
02-09-2011, 01:44 PM   #12
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QuoteOriginally posted by pxpaulx Quote
Yes, they have a different apparent DOF on different sized sensors, but an F4 lens is still slower than an F2.8 lens any day of the week.
Not "any day of the week", because the sensor size matters when you talk about equivalent images.

Yes, an f/2.8 lens stays an f/2.8 not matter on which camera, but at the same time it doesn't change its focal length either.

If you convert focal lengths between formats, you must do the same for f-ratios. Such conversions are useful to understand which lens would be needed on a different format to get equivalent images.

With a crop factor of 1.54 (K-7), for instance, we see that an APS-C 35mm f/2.4 lens (DA-L 35/2.4) only requires a 54mm f/3.7 on FF. If you put the 35/2.4 lens on an FF camera, you get faster speed but also a different FOV.

QuoteOriginally posted by CarbonR Quote
A quick comparison I made with my Canon 5D (24x36) and my Pentax K100D (APS-C). Same lens (Helios 40, 85/1.5 @f/1.5), same reproduction ratio :
I don't know why you had to increase/reduce contrast. At this level, the main difference we see are different exposures. With less exposure the K100D could have handled the highlights better and with this scene there would not have been any noticeable noise because of the necessary push in post.

QuoteOriginally posted by CarbonR Quote
We can also see a loss in lens' caract (swirly bokeh), ...
This might be just an effect of the different focus distance, not the different sensor format (of course the two are somewhat linked).

QuoteOriginally posted by CarbonR Quote
I think the guys who say APS-C cams are lighter/more compact does not compare them at equivalent dof.
I'm not sure. FF lenses need bigger image circles and this either requires bigger glass or increased magnification of the image circle. Both these measures require glass.

I don't think one can compare lens prices across different formats. The price does not directly reflect the manufacturing cost but what the market is prepared to pay for a particular product.

I understand that while the larger image circle adds to the cost of FF lenses, the lesser requirements in terms of resolution and precision assembly make them cheaper to produce as a net effect. If you wanted to get an FF lens that is as good as an APS-C lens at the pixel level (identical pixel-pitch between formats assumed) then I think the FF lens would be more expensive. If you want an FF lens that performs equivalently on the image level, it can be made cheaper than a corresponding APS-C lens.

QuoteOriginally posted by dlacouture Quote
Anyway, as full-frame cameras usually have easily a 1 or 2 ISO stops advantage over APS-C, I feel the conversion is quite right.
Currently, with APS-C sensors like that in the K-5, there is no such ISO stop advantage for FF cameras. They may gain it back with the next generation of FF sensors, but we are talking about sensor generations here, not format differences. There is a format difference which is "dynamic range", but there is no "low light" difference between formats.

Last edited by Class A; 02-09-2011 at 02:37 PM.
02-09-2011, 02:07 PM   #13
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QuoteOriginally posted by Class A Quote
Currently, with APS-C sensors like that in the K-5, there is no such ISO stop advantage for FF cameras. They may gain it back with the next generation of FF sensors, but we are talking about sensor generations here, not format differences. There is a format difference which is "dynamic range", but there is no "low light" difference between formats.
Yes, because the current FF cameras still have the previous generation sensors compared to the K5/D7000.

And even using the same sensor generation and same pixel pitch, FF would still have an advantage as you'd have twice the pixel count for the same image, thus obtaining better high-iso images (for the same output size, of course).
02-09-2011, 02:33 PM   #14
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QuoteOriginally posted by dlacouture Quote
And even using the same sensor generation and same pixel pitch, FF would still have an advantage as you'd have twice the pixel count for the same image, thus obtaining better high-iso images (for the same output size, of course).
You mean "better" as in "finer-grained noise"? That's true.
02-09-2011, 04:09 PM   #15
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IMO only important advantage of full frame is wide angle at short focal lengths. All the rest is technical back and forth and changes with every generation. 1.5X crop is approaching 20MP.
Also IMO, the disadvantage of APS-C is not the sensor but the lenses. They won't work on FF. I wish they never put crop sensors in 35mm style DSLRs

I think it might have been because of the high cost of the then superior CCD. CMOS are now just as good or better. I think (hope) FF format will win now that a FF DSLR can be had for 1/4 the price they were with the old CCD.
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