[regor]

F4 is F4, but that doesn't say much. Faster is a difference in time. If you don't mind (or even like) the shallower DOF, you can generally get away with setting the ISO twice as high on a fullframe camera, so you will need only half the shutter speed.

So, IMHO, the "speed" of a lens depends on more than the maximum aperture of that lens.

To add to your point. F4 is F4 and the DOF at F4 at a given FL is always the same on FF as it is on APS-C. What changes, and the only thing that changes, is the Field of View. To say that DOF is shallower on FF is a mis-conception. What causes the apparent "greater DOF" of APS-C is the use of a shorter FL on APS-C necessary to get the equivalent FOV obtained on FF at the same distance from the subject. Crop a FF image to the FOV of APS-C, they will have exactly the same DOF. Walk away from the subject with an APS-C subject to obtain the same FOV at the same FL as FF, and the DOF will still be exactly the same; however, the perspective will be different, hence the only difference between FF and APS-C is the combination of DOF for a given perspective (which is controlled by distance from the subject). In theory, take portrait as an example, a full head image taken with a 70mm on APS-C, will be better than the same image taken with a FF camera as you need be closer to the subject, hence introducing more distortion ! In this case, APS-C is actually better than FF as you can work further away from the subject ! Work from the same distance with a FF + 105mm lens, and you'll end up with the same perspective with less DOF and less resolution perhaps (perhaps too little even). So which is best ? Neither, they are just different, and you learn to work with the differences !

addendum:

Conclusion: At equal Field of view, you could never reproduce the qualities of a 105mm + FF with and APS-C sensor, nor could your reproduce the qualities of a 70mm + APS-C with a FF sensor ! What advantage FF has comes with the design of the 50mm FL, which for some reason happens to have the highest resolution of all FLs. The combination of FOV + DOF + RESOLUTION which is optimum at the same magnification as the human eye makes it a winning combination.

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Last edited by regor; 09-27-2013 at 03:13 PM. Reason: typo and clarification

[Poit]

The K3 isn't looks like the pictures. It's a little bigger, and heavier than the K5.
The body will be the same as the FF Pentax in 2014.
The k3 grip is not the same as K7-K5 grip.
The new AF is incredibly better than K5.
Double Sd slot, and many more improvements.
Less than two weeks......

Every part of my photographer-being wants this to be an accurate statement.

Well, the 'less than two weeks' part seems to have been confirmed by the SRS event (posted after this was).

The 'same body as FF' makes a lot of sense too (streamline the manufacturing, homogenise the user interface / experience, visually align the top-end systems, make the upgrade from APS-C to FF a 'Pentax' experience).

I'm really hoping the 'incredibly better' AF, and double SD slot predictions are true as well.

I am daring myself to believe this will be an awesome new camera

[robjmitchell]

Yes I agree. I'm just trying to figure out why all things being equal in exposure settings people say A lens is faster on FF

Those FF lenses had Red stripes that made them go faster!

[nanok]

True, but: a given focal length will produce different perspectives on fullframe and APS-C.

no. please, perspective depends _only_ on distance to subject, not on lens, not on format or alignment of planets. lens specific distorsions are not part of perspective (they are part of optical aberrations )

So you adjust focal length to e.g. 200mm on fullframe and 135mm on APS-C.

about right, but this is because of field of view.

But now the DOF is shallower on FF. So you stop down to F5.6 on FF while you keep F4 on APS-C.

not entirely sure, but an online dof calculator could clarify that (goes and checks)

nikon aps-c, 135mm f/4, 4m subject distance:

Depth of field
Near limit 3.93 m
Far limit 4.07 m
Total 0.14 m

In front of subject 0.07 m (49%)
Behind subject 0.07 m (51%)

Hyperfocal distance 227.9 m
Circle of confusion 0.02 mm

nikon d800 &co, 200mm, f/4, 4m

Subject distance 4 m

Depth of field
Near limit 3.94 m
Far limit 4.07 m
Total 0.13 m

In front of subject 0.06 m (49%)
Behind subject 0.07 m (51%)

Hyperfocal distance 235.9 m
Circle of confusion 0.03 mm

so pretty much spot on (courtesy of dofmaster.com)

To be able to use the same shutter speed, you increase ISO to 200 on FF while keeping it at 100 for APS-C. The FF sensor is larger anyway, so it can cope better with noise.

i wouldn't bet on that. this has been the case so far, but keep in mind much larger sensors are notoriously bad at higher iso's (medium format).

However, the difference between F4 and F5.6 is approximately 1.4 (square root of 2).

no. the ratio of light gathering between each "well known" apertures is 1/2, that's precisely why they are well known, and if you want to verify, you need to square them:

5.6^2/4^2
1.96000000000000000000

But APS-C sensors are more than 1.4 times smaller than FF sensors. In case of Pentax, it's a 1.5x crop.

we're mixing units here. 1.5x crop is a linear measurement, light gathering capabilities are a factor of surface (so linear squared, one might say, except squaring the crop factor is inaccurate unless you're talking about a rectangular frame). in short, assuming the same aspect ratio, the "35mm" sensor is twice the surface of aps-c, and this is why it's usually "one f-stop away" in nearly everything (like dof at same field of view). this is also why f-numbers need to be squared to obtain the exposure ratio (they are a linear ratio between focal length and aperture size)

So F5.6 on FF will still give a slightly shallower DOF than F4 on APS-C, given equivalent focal lengths of 200 and 135mm respectively. And noise performance on the FF sensor at ISO 200 will be slightly better than the APS-C sensor at ISO 100, assuming the sensor technology is comparable.

dof: no, as you see from above, it's pretty much the same (close enough it makes no difference)
comparing noise performance of ff at iso200 to aps-c at iso100 on modern sensors is a purely academic exercises imho.

However, this point is moot, since we were trying to equalize the DOF. So although the FF performance is slightly better, its DOF will still be a little shallower as well.

A few cases where fullframe is better than APS-C:
- The detail level at the same ISO levels
- Ability to create a shallower DOF at wide viewing angles with the same F-number
- Ability to shoot in darker environments by using higher ISO's and the same F-number if you don't mind (or are actually aiming for) the shallower DOF

all these "all else being equal" comparisons have two problems: all else is never equal, and comparative discussions might not be meaningful for practical purposes from a point on (for instance, comparing the output of a sensor like the one in the k-5 with one in a "comparable" 35mm camera, using good quality (read: excellent) glass, one might be surprised how close they will be). Technology is at a point where one no longer _needs_ bigger sensors for image quality purposes (exceptions are rare, and tend to drift towards phaseone and the likes anyhow), you might _want_ it for a bigger, brighter viewfinder, you might dream of it because you have some special pixie dust lens which only really works "properly" on a 35mm frame, or just because you can afford it. but need it? nope. sensor technology has reached "maturity" a few years ago (about the time k-5 was released), in other words. that's not to say improvements are no longer possible, but rather that you probbaly don't need them

now i should run...

ps: sorry for the rant, nothing personal, i have a compulsive reaction to misuse of the term "perspective" in particular

[starbase218]

To add to your point. F4 is F4 and the DOF at F4 at a given FL is always the same on FF as it is on APS-C. What changes, and the only thing that changes, is the Field of View. To say that DOF is shallower on FF is a mis-conception. What causes the apparent "greater DOF" of APS-C is the use of a shorter FL on APS-C necessary to get the equivalent FOV obtained on FF at the same distance from the subject.

You are right of course. I also stated that in my previous post. But I didn't want to reiterate on it again.

Crop a FF image to the FOV of APS-C, they will have exactly the same DOF. Walk away from the subject with an APS-C subject to obtain the same FOV at the same FL as FF, and the DOF will still be exactly the same; however, the perspective will be different, hence the only difference between FF and APS-C is the combination of DOF for a given perspective (which is controlled by distance from the subject).

Not entirely. Subject distance also affects DOF. The greater the subject distance, the greater the DOF. This is actually why I don't see much use for fullframe in macro photography. Due to the short subject distance, you almost always want to stop down to get more of your subject in focus. On APS-C you don't have to stop down as much.

In theory, take portrait as an example, a full head image taken with a 70mm on APS-C, will be better than the same image taken with a FF camera as you need be closer to the subject, hence introducing more distortion ! In this case, APS-C is actually better than FF as you can work further away from the subject !

True if you treat focal length as a constant, which I did not.

Work from the same distance with a FF + 105mm lens, and you'll end up with the same perspective with less DOF and less resolution perhaps (perhaps too little even). So which is best ? Neither, they are just different, and you learn to work with the differences !

I don't think it would have less resolution. On the contrary, having less DOF would mean using a greater relative aperture. This would allow you to use lower ISO values, thereby lowering noise and allowing the fullframe sensor to capture more detail.

[nanok]

To add to your point. F4 is F4 and the DOF at F4 at a given FL is always the same on FF as it is on APS-C. What changes, and the only thing that changes, is the Field of View. To say that DOF is shallower on FF is a mis-conception. What causes the apparent "greater DOF" of APS-C is the use of a shorter FL on APS-C necessary to get the equivalent FOV obtained on FF at the same distance from the subject. Crop a FF image to the FOV of APS-C, they will have exactly the same DOF. Walk away from the subject with an APS-C subject to obtain the same FOV at the same FL as FF, and the DOF will still be exactly the same; however, the perspective will be different, hence the only difference between FF and APS-C is the combination of DOF for a given perspective (which is controlled by distance from the subject). In theory, take portrait as an example, a full head image taken with a 70mm on APS-C, will be better than the same image taken with a FF camera as you need be closer to the subject, hence introducing more distortion ! In this case, APS-C is actually better than FF as you can work further away from the subject ! Work from the same distance with a FF + 105mm lens, and you'll end up with the same perspective with less DOF and less resolution perhaps (perhaps too little even). So which is best ? Neither, they are just different, and you learn to work with the differences !

while this is a much saner aproach to this discussion , it's still not entirely correct: in short, dof at the same field of view and the same aperture between two different fromats will be less on the bigger format, while dof with the same lens at the same aperture on both fromats will be _less_ on the smaller format.

of course, the crop example is perfectly correct, it's the first phrase i have an issue with . the "walk away" example is a whole different can of worms imho, as you mentioned, this changes perspective, so it's not even really relevant if what you noted is accurate (because we're talking about a whole new picture already, so you have to re-think everything from scratch)

[nanok]

Yes I agree. I'm just trying to figure out why all things being equal in exposure settings people say A lens is faster on FF

strictly speaking, because it's true (mostly because of physics: when you crop the image of the lens with the frame of 24x36mm, you get x light on your photosensitive area, altogether, if you crop the same image with the aps-c frame, you get, to make a long story short, x/2 light altogehter, so one stop). it also "happens" that dof at the same field of view is also one stop away. so "a lens of the same f for the same purpose on the two formats is "faster" on one than the other, like: a 35/1.4 is _not_ equivalent to a 50/1.4 on 35mm, but rather to a 50/1.8 or so. bleh

[Andi Lo]

dof: no, as you see from above, it's pretty much the same (close enough it makes no difference)
comparing noise performance of ff at iso200 to aps-c at iso100 on modern sensors is a purely academic exercises imho.

Except that in FF you're not stuck with f 5.6. There's the option of f/4 - F2.8 for zooms, and down to f/1.8 with reasonably priced) primes. The APS -DOF equivalent of pedestrian FF lenses are glass with some exotic specs: 24-105/4 (=17-70/2.8), Standard 28-75 (=18-50/2), 35/2 (=24/1.4) or heck, the super cheap 50mm/1.4 (30/1)

Also, many, many cheap FF lenses are usually have very acceptable performance wide open at the centres, which for wide open shots are usually not an issue. I suspect that FA 50 will actually take a new life at f/1.4 in FF; the lens doesnt need to resolve as many pixels as with a 16mp APS, as long as the FF sensor is under 30mp.

I feel horrible that I've only been posting FF stuff in the k-3 thread. I'm really excited at the prospect of all-cross point AF, if it's true it might indeed be the best pentax to date, and if the tracking is good (I'm hopeful but doubtlful at the same time) we might get all those potential d400 buyers.

[derekkite]

Isn't the advantage of a larger sensor more apparent when you want wide angle? To get wide with aps-c you need 12-15mm, with all the inherent distortions that come from such a lens. With full frame you are somewhere in the 20-30mm range, with MF quite a bit longer. Of course for telephoto it works the other way; a full non cropped exposure at 300mm on all three will give you a quite a different image.

The dream machine for landscape has a large sensor. From there on you deal with compromises. Each size has it's advantages and disadvantages, and depending on what and how you shoot, they are more or less important to you.

What is the size of the aperture on an aps-c 15mm at f22 compared to a wide angle medium format lens at f22? Or a full frame one? Again, it depends how and what you are shooting.

[nanok]

Except that in FF you're not stuck with f 5.6. There's the option of f/4 - F2.8 for zooms, and down to f/1.8 with reasonably priced) primes. The APS -DOF equivalent of pedestrian FF lenses are glass with some exotic specs: 24-105/4 (=17-70/2.8), Standard 28-75 (=18-50/2), 35/2 (=24/1.4) or heck, the super cheap 50mm/1.4 (30/1)

Also, many, many cheap FF lenses are usually have very acceptable performance wide open at the centres, which for wide open shots are usually not an issue. I suspect that FA 50 will actually take a new life at f/1.4 in FF; the lens doesnt need to resolve as many pixels as with a 16mp APS, as long as the FF sensor is under 30mp.

I feel horrible that I've only been posting FF stuff in the k-3 thread. I'm really excited at the prospect of all-cross point AF, if it's true it might indeed be the best pentax to date, and if the tracking is good (I'm hopeful but doubtlful at the same time) we might get all those potential d400 buyers.

this is a long discussion, of course you might do such calculations; sigma has answered finally with the reasonable solution: aps-c is a format of it's own, and there's no physical reason why such "equivalent" lens should not be possible. I expect more of those from them.

personally i like the slight dof increase from aps-c, it's "just right" for most purposes. it's a matter of usecase of course. i do lust for the bigger viewfinder, every time i raise my old pentax film camera to my eye, but i recall seeing a canon 5d a few years ago, i was terrified. that thing is huge (and it's one of the smallest). (if pentax manages to cram a 35mm sensor in a k-5 like body.. there will be thunder; note that even the "tiny" nikon d7000 looks huge aside the k-5)

i agree, k-3 sounds like a sweet little camera. not surprising, i can't recall a time in the past 5 years at least when pentax has released a new model and has failed to make everybody drool. but let's see it first

[filoxophy]

Yeesh, the whole equivalence thing comes up here almost as much as FF! How many more angles and analogies can possibly be left?

[nanok]

Isn't the advantage of a larger sensor more apparent when you want wide angle? To get wide with aps-c you need 12-15mm, with all the inherent distortions that come from such a lens. With full frame you are somewhere in the 20-30mm range, with MF quite a bit longer. Of course for telephoto it works the other way; a full non cropped exposure at 300mm on all three will give you a quite a different image.

actually, no. there's no physical reason why obtaining the same field of view on different formats will be harder on one format as opposed to another, that i know of at least. there's no direct limitation to mm of focal length, the major problem is how much of "spherical" reality you try to flatten down on a flat sensor/print/screen, that's where the "distorsion" comes from.

The dream machine for landscape has a large sensor. From there on you deal with compromises. Each size has it's advantages and disadvantages, and depending on what and how you shoot, they are more or less important to you.

maybe. for landscape, the dof penalty of larger formats even at wide angles is so painful that you _must_ use camera movements to get your shot (yes, it's cool, unique, etc, but remember: you _must_, it's not that "you can if you want")

What is the size of the aperture on an aps-c 15mm at f22 compared to a wide angle medium format lens at f22? Or a full frame one? Again, it depends how and what you are shooting.

that's the only "physical" worry, but then again, it's "okay": you can obtain the same dof on aps-c one stop down, so diffraction, although a bigger problem on the smaller fromat, should be alleviated by the increased dof (not having to close the aperture that much). by deffinition, f/22 at 15mm is 15/22

15/22
.68181818181818181818
15/16
.93750000000000000000
15*1.5/22
1.02272727272727272727

so there's a slight advantage to 35mm it seems, at a quick glance (physical aperture a bit bigger for same fov and dof), that's not surprising.

[Wired]

strictly speaking, because it's true (mostly because of physics: when you crop the image of the lens with the frame of 24x36mm, you get x light on your photosensitive area, altogether, if you crop the same image with the aps-c frame, you get, to make a long story short, x/2 light altogehter, so one stop). it also "happens" that dof at the same field of view is also one stop away. so "a lens of the same f for the same purpose on the two formats is "faster" on one than the other, like: a 35/1.4 is _not_ equivalent to a 50/1.4 on 35mm, but rather to a 50/1.8 or so. bleh

Ok, but is the overall exposure brighter?

Hypothetically if you take a Q, EM5, K5, and D800 and have lenses that recreate the same field of view, and all are set for f8. if you set the iso to say 100, shutter to 1/250" isn't the exposure going to be the same? I haven't actually tested this but just from using a light meter I understand this is the basic principle.

[nanok]

Yeesh, the whole equivalence thing comes up here almost as much as FF! How many more angles and analogies can possibly be left?

yeah, good point. isn't there an faq or something already?

[nanok]

Ok, but is the overall exposure brighter?

Hypothetically if you take a Q, EM5, K5, and D800 and have lenses that recreate the same field of view, and all are set for f8. if you set the iso to say 100, shutter to 1/250" isn't the exposure going to be the same? I haven't actually tested this but just from using a light meter I understand this is the basic principle.

you will get the same expousre, by definition, yes. (this is the whole point of f-stop, iso, and shutter speed -- though shutter speed does not realy need any "help" to be transferred over formats)

edit: this might seem confusing, but think of the previous example again (croping from a "conveniently large frame"): imagine it's a 6x6 cm frame, will exposure change if i take my scisors out and crop it? of course not. you might say exposure on the photosensitive are is a facot of how much light per square inch you get, not on how much light you get in all.