[stevebrot]

Thanks for all the responses! I have to go to work now so shall take all this in better upon my return. I just wanted to point this out, that for the XS 40mm, there is no Field of View for Full-Frame listed.

There are online calculators that allow comparison between various formats. The interface is a little (translation VERY) quirky (subject distance is irrelevant for FOV), but I like this one...

Camera Field of View Calculator (FOV)

Fill in focal length and format size (line labeled 1) and click the "calculate..." button.

Language is sometimes a key. They don't call APS-C and smaller "cropped format" for nothing and a crop is nothing more than just what you might have had trimmed to suit. As for difference between designated APS-C lenses (DA in Pentaxland and DX over in Nikonville) and those designated as FF (K, A, F, FA, D FA in Pentaxland and FX in Nikonville), the image circle cast from the rear of the lens matches the sensor/film size for each format.


Steve

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Last edited by stevebrot; 11-26-2017 at 03:06 PM.

[Na Horuk]

You missed out the qualification in bold italics. You're welcome

Doh! Thanks

[mikesbike]

Well what makes a 40mm, or a 50mm FL relevant and meaningful, as such each of them is on full-frame body, just as any other focal length, essentially is its field of view. And the same goes for APS-C. So if you want more telephoto on a FF body you go to a lens having a narrower FOV. Right.

If you want the same FOV with APS-C as you have with a lens of a given FL on a FF body, you have to go to a lens of a different FL- a shorter one.

So if you put that 40mm lens on an APS-C body, it will have a narrower FOV, therefore it will essentially be more telephoto than it is on a FF body. The factor is about 1.5x, which will present a FOV like a 60mm lens on the FF body. When you take the picture the way you want it framed, you will not be discarding any of it afterward. To get that same picture with a FF body and the same lens, you will either have to switch it to the APS-C FOV (if it can do that), or after the image is taken crop the additional FOV the lens provides through the FF format to match the FOV obtained by the lens with APS-C, discarding part of the picture taken through the lens. By discarding, you are throwing away pixels- resolution. If the FF sensor is of high enough in MP design, so that after what is eliminated or discarded then matches the pixels of the APS-C image, the two will be virtually identical in resolution and content.

Even though technically the DA designation indicates lenses with glass elements cut in width to match APS-C sensors, therefore too narrow to use with a wider FF sensor, a few DA lenses are in fact cut wide enough for FF use. Some of these are basically upgrades of old 35mm film lens designs and thus will work perfectly well with a FF body.

[BWG]

Another article explaining crop factor. Cambridge in Colour is excellent.

Digital Camera Sensor Sizes: How it Influences Your Photography

[stevebrot]

So if you want more telephoto on a FF body you go to a lens having a narrower FOV. Right.

Ummm...not exactly. If one wants narrower FOV at a specific focal length than with a FF camera, use a crop sensor camera. Note that the magnification to the sensor is identical for both formats. FOV for a given magnification is a side-effect of format size and nothing more.

Conversely, if one wants a narrower FOV for a particular format (e.g. 24x36mm), increase magnification with a longer focal length. FOV for a given format is a side-effect of magnification and nothing more.

("More telephoto" is probably not a good term. Telephoto is a specific design feature that is common in longer lenses, but not all long lenses are telephoto and not all telephoto lenses are long focal length. A lens is either telephoto or it is not. Wikipedia: Telephoto lens)

[stevebrot]

Conversely, if one wants a narrower FOV for a particular format (e.g. 24x36mm), increase magnification with a longer focal length. FOV for a given format is a side-effect of magnification and nothing more.

After posting this, it occurred to me that the most obvious example is a pair of binoculars where magnification and FOV are completely separate considerations.


Steve

[Culture]

So basically its like looking through two pipes.
Both are 50 cmm long. One is 10 cm wide and the other 20 cm.
You see more with 20 cm but they are the same length that is why focal length is not important.

I think the reason why people say you get a short telephoto or extra reach is because you dont have to crop much with an aps-c. That is filling the frame and getting rid of empty space as compared to ff.

Extra reach should mean more focal length. But they still say its the same focal length. that is where the confusion comes i guess.

[UncleVanya]

So basically its like looking through two pipes.
Both are 50 cmm long. One is 10 cm wide and the other 20 cm.
You see more with 20 cm but they are the same length that is why focal length is not important.

I think the reason why people say you get a short telephoto or extra reach is because you dont have to crop much with an aps-c. That is filling the frame and getting rid of empty space as compared to ff.

Extra reach should mean more focal length. But they still say its the same focal length. that is where the confusion comes i guess.

The density of pixels also impacts things. As Sandy said let's try to not get too deep in the weeds here - but density matters. This is why the Q gives such enormous reach with modest focal length lenses.

[biz-engineer]

The red is FF, the blue is APS-C...

What format is the circle? :-)

[BruceBanner]

Well... I would just like to say big thanks to everyone who put their 2 cents in and tried to help poor Bruce. A few members here said a few things that resonated the most with me, others said stuff I already knew but yet either the penny didn't drop properly or wasn't quite helpful in dragging my brain over to see things differently/correctly.

Here's where my experience with photography gives way to the problem in my brain. I consider myself to be a newbie to photography. My first a K-50 and then within a short space of time (1yr or so) moving onto the K-1. I identified a lens with a certain mm as a field of view from which to gauge what it would look like to shoot through. 50mm portrait, 100mm tele, 15mm wide etc. But really that was kinda my first mistake in brainwashing my brain because fair enough, a 40mm lens is always a 40mm lens, but lenses don't take the photo from where you're standing at, the camera does (with the lens attached obviously). Take the same lens (say the FA 50mm 1.4), set tripod up, mount the K-1, slap the lens on, take the pic and the shot is 'wideish'. Dismount the K-1, slap the KP on, mount the same FA 50mm 1.4, take the shot again, and now the photo is different, appears narrower and missing scenery that the K-1 with that combo got. Why is that important? Because when you're shopping around looking for wider lenses or narrower ones the mm isn't enough information, plain and simple. If I had a full frame manual 24mm, I might think a DA 15mm would be a lot wider... but actually maybe not that much in reality.

Yes the 50mm is 50mm, but where I've been going wrong is that for months I was shooting with a DA 50mm 1.8 on my K-50 (APS-C: 31.5 ° / 27 ° ), I thought this was what 50mm looked like (and indeed it does on an asp-c sensor), but if I wanted to recreate that look/field of view on the K-1, it wasn't a 50mm I was seeking, it should have actually been a Fa 77mm 1.8. (Full frame: 31.5 ° / 26 °). I almost feel like the DA 50mm should have additional text on the lens, something like 77mm equivalent FoV lol.

Likewise, if I enjoyed the perspective that a FA50mm gave on the full frame K-1, and I wanted that perspective on the KP, I shouldn't be looking for a 50mm but rather perhaps something like the DA35mm as it has a more closer field of view relationship.

Another example. If I stand on a hill and set a tripod up, if I get my K-1 out and mount it first, then I slap on a FA 20mm 2.8. it's actually going to capture more of the scene/landscape than if I pop my K-1 out and slap in the KP with the DA15mm attached! That kinda goes against how my brain has been brainwashed (all by myself... no one else to blame here lol).

That for me has made the penny drop, I can no longer just look at mm for mm sake, it's kinda irrelevant information to me as the lens itself doesn't take the picture, it's what it's connected to. I natively expect a lower mm to correspond to a wider shot, but that clearly isn't the case.

Another interesting thing to get my head around is that although the K-1 has the crop mode (ie same perspective as the KP when mounted in the same position using that mode), the crop mode gives only 15-16mp or something like that in crop mode, whereas the KP would give me the full 24mp of that exact same capture. Perhaps its arguable that resolution is not everything and that the K-1 will still do a better job with the 15-16mp over the KP's 24mp.. I dunno.

I guess ff lenses are great because they are versatile, working with both FF and ASP-C sensor cameras, ASP-C intended lenses you have to be careful, as they may not provide the same flexibility as FF lenses. There are exceptions (DFA 100mm, DA 40mm, DA 50mm etc) but yeh especially the wider angled ASP-C lenses are more likely to run into vignetting and therefore less versatile.

Someone mentioned here that film cameras etc started off commonly with 35mm film, and thus we also had 35mm sensors (but they were expensive so the digital world started doing smaller sensors etc), what I'm trying to get at is when we talk of 50mm lens (in general) are we therefore agreeing more with a viewing angle of 47 / 40, because this has been the angle/field of view most of us here (history) have experienced and come to think of? Because if not... a 50mm on a ASP-C sensor (such as the DA 50mm 1.8) gives a far more narrower view of 31.5 / 27... It's almost as if we should ditch the mm entirely and just talk of FoV, have that slapped on the lenses, if it's a lens that can be used on both FF and ASP-C then have that info on there, if only one sensor is applicable then only one FoV. I can't help feel that if things were set out like that I might have perhaps made less purchasing errors or got my head around this better.

In answer to my thread subject, then yeah... I guess a DA 40mm F2.8 XS (which was intended for ASP-C) actually really is more like a 60mm field of view if coming from the FF world i.e. you own a FF camera use a 40mm, and then slap same lens on a new ASP-C camera, that view now looks more like how a 60mm lens would look on a FF... (I think). lol

Basically when future shopping I need to pay more attention to the lens and what it will sit on. I currently have the DA15mm, it sits mostly on the KP, if I buy the Irix 15mm f2.4 blackstone and attach it to the K-1 it's going to give me a much wider perspective (I think the same as what the DA 15mm looks like in ff mode on the K-1 but just minus the vignetting?), in some ways I still kinda feel like I don't know what a 15mm shot really looks like, because the DA15mm is only able to be used on ASP-C successfully and in my mind I am kinda using full frame as the bench mark when talking mm and viewing angles etc.


Wow... just wow...

[UncleVanya]

And that is why equivalence is useful and the devil's tool - all at the same time!

[BruceBanner]

I have the FA 50mm 1.4, why bother buying the FA77, just whack the FA 50mm on the KP and get a similar FoV! heh.

[Sandy Hancock]

I have the FA 50mm 1.4, why bother buying the FA77, just whack the FA 50mm on the KP and get a similar FoV! heh.

Fair enough. Before the K-1 was released it was more or less what we all did if we wanted that specific field of view. But the FA77 renders differently from the FA50/1.4, handles better, is sharper at wide apertures, and is prettier.

[Alex645]

Bruce, Lots of good posts above explaining all this. I'll try to keep it simple:

FF lenses are designed to create an image circle large enough for the 36x24mm sensor so it won't vignette. You can use these lenses on an APS-C sensor, as the sensor is smaller, and if anything, benefit from the qualities usually found in the center of lenses. The trade off is usually cost and size.

APS-C lenses are designed to create a smaller image circle for the 22.2x14.8 mm, which generally allows the lenses to be smaller and less expensive. If you use one of these on a FF sensor, you will usually see vignetting in the corners and significant optical flaws toward the edges. There are exceptions at different apertures, etc.

The field of view equivalence between APS-C (Pentax, Nikon, Sony; not Canon) and FF is 1.5x. So a 60mm field of view in a K-1 will look similar to a 40mm field of view on a K-50. If you put a 40mm on your K-1, it would look similar to approx. 27mm on a K-50.

What you see with the 18-55mm on a K-50 would look similar with a 28-85mm on a K-1. I always think of what are considered normal for both formats. Back in the film days, the 35mm film (36x24mm=FF) SLRs came with 50mm primes. Purists will argue it's really 43mm or 58mm, but let's just call it 50mm. Focal lengths less are wide angle and greater are telephoto.

So for APS-C, using the 1.5x factor, a normal lens would be 35mm. Again, some will swear by 31mm and others 40mm. So for APS-C consider less than 35mm as wide and greater than as telephoto.

When we say "focal length is focal length" or "40mm is 40mm", that's because it's true in that the focal length doesn't magically change just because a different format is cropping the image circle into a different sized rectangle. But as @boriscleto has posted, the field of view captured will be different with the same focal length but different sized sensors.

---------- Post added 11-27-17 at 02:49 AM ----------

I have the FA 50mm 1.4, why bother buying the FA77, just whack the FA 50mm on the KP and get a similar FoV! heh.

Yes and no. The FoV will be similar, but the rendering and the bokeh may be quite different. My 645 MF 75mm f/2.8 gives me superior results to my FF 50mm f/1.4 although they have similar FoV. And I prefer the aesthetics of my 50mm f/1.4 on FF over my 35mm on APS-C. Such is the price for pixie dust.

[BruceBanner]

Fair enough. Before the K-1 was released it was more or less what we all did if we wanted that specific field of view. But the FA77 renders differently from the FA50/1.4, handles better, is sharper at wide apertures, and is prettier.

Yeh you're quite right, can't really compare like that (was being a little facetious), but for us on a budget it can help us round out our gear and what's lacking in terms of capturing an overall FoV. Having the KP and the K-1 now gives me a few more options when dual shooting with primes. I noticed for example on the weekend how lovely the DA 40mm XS was on the K-1, it's a really nice dual or trio portrait shooter. Not quite the same story when sitting on the KP.

I must make a list of my lenses and the corresponding FoV combos they give on different cameras hehehe.

But yeh, I've actually had my eye on that 77mm for awhile, it looks damn impressive, as does the FA 85mm, agree that they are completely different beasts entirely =)