[jolepp]

...
Purchasing a modern 50mm which works with smaller censors will yield a 'wider' photographic view which will actually be 50mm?

Nope, a modern 50mm will be the same as far as the view is concerned. The thing is that the APS-C DLSR manufacturers decided to set things up so that the existing film lenses would work. If they had changed the lens spec to start with then it would be a different matter: old film lenses wouldn't work directly and the initial selection of lenses would have been small.

[chrisrussell]

Thanks Lowell - appreciated.

So what are the cons for using standard 50mm on digital with smaller sensors compared with film?

[jimH]

So essentially when I shoot with my FA 50mm f1.4 on my Pentax K-x - I'm actually shooing with a tighter 75mm equivalent but with all the bells and whistles of a 50mm.

Purchasing a modern 50mm which works with smaller censors will yield a 'wider' photographic view which will actually be 50mm?

No! purchasing a modern 50mm lens will give you exactly the same field of view as the FA 50 or any other 50 mm lens for that matter. If you want the equivalent field of view of a 50mm in 35mm film format, on your K-x camera, then you would have to get a 35 mm lens.

[Lowell Goudge]

Thanks Lowell - appreciated.

So what are the cons for using standard 50mm on digital with smaller sensors compared with film?

Really and this is now not really related to crop factor at all, but lens and camera design, there are several issues pertaining to old glass and new cameras. Please forgive me if this gets a little technical, at least in concepts, because I know no other way to explain it.

1) Internal reflections

There have been some reports about reflections off the sensor, and the back element of the lens, causing reduced contrast or bright spots on the images recorded. this did not occur on film because the emulsion was much less reflective than the UV filter infront of the sensor. Personally I have not seen this with any of my 20+ film lenses.

There are also cases where there have been reflections off shiny metal parts on film lenses, or adaptors causing similar problems. I have experienced this on one lens only, and I corrected it by using flat black paint on the offending part (lens mounting ring for the rear element)

Newer digital lenses have coatings on the rear elements to minimize reflections

2) uneven exposure due to the angle of incidence of the light from the lens to sensor.

There are several different issues with digital sensors, compared to film.

For digital sensors, and this I believe is repored to be more of an issue with C-MOS than CCD sensors, there is a belief that the "deep wells" of the sensor require the light to hit the sensor closer to perpendicular angles than on film. Light at low angles was absorbed by the side walls of the lens, causing reduced esposure. This was reported as an issue with large apertures (i.e. F1.2 and 1.4 specifically) and wide angle lenses.

A second issue also related to the angle of incidence of the light on the sensor was that the micro lenses on the individual sensors causes some light to simply reflect off the lens as opposed to entering the sensor proper. Again this was largely attributed to wide angle lenses and large apertures.

I have not really seen significant evidence that this is a problem, but I have seen slight vignetting of my K50 and Super Takumar 50 at F1.4. The vignetting is about 3/4 of one stop which is hardly noticible, is only at the corners of the frame, and is gone totally by F2. From some shots posted here, the FA50 behaves the same way, but again, you need to actually measure the histogram across the entire frame, using a photo editor to see this. In practice it goes un, noticed.

I have not really seen any issues of undue vignetting with my 24 and 28mm lenses, but have not gone looking for them.

Newer digital lenses have designs that attempt to have the light hit the sensor closer to perpendicular, but are not totally free from some slight vignetting wide open either.

3) lens hoods.

Due to the difference in field of view between film and digital (ASP-C) lens hoods on film lenses are designed to provide protection for the field of view on film. As a result, they are not optimized for Digital, and better Longer hoods could be used to provide maximum benefit. Using lens hoods does improve greatly the contrast on most lenses, therefore, getting better hoods for film lenses could be considered.

[Manel Brand]

Thanks Lowell - appreciated.

So what are the cons for using standard 50mm on digital with smaller sensors compared with film?

Yes. You have to step back.

[chrisrussell]

and therefore lose resolution?

[sterretje]

Maybe the above explains it.

I have taken an arbitrary lens (coincidence that it is 50mm). The solid thick red line at the left is the film and the subject is at the right.
The thinner red diagonal lines show which part of the subject will get on the film (part of the foreground, the full tree and part of the sky).

With an APSc camera, the sensor is smaller (the solid thick blue line at the left of the lens). What is caught on the sensor is therefore a smaller part of the subject and indicated by the thinner blue diagonal lines; in this example only the tree, no foreground or sky).

When we extend the thinner blue lines to the left (dash dot), we will get to a point where the height of the image has the same height as the original height on the film (the dashed thick red vertical line).

Simple (or not so simple) maths will reveal that this point is at

Code:

focal_length * height_of_film / height_of_sensor

Filing in the numbers we get 50 * 24 / 16 = 75mm and therefore it is said that a 50mm lens on APSc has the equivalent field of view of a 75mm lens on film

Hope this helps

PS film refers to 135 film as used in SLRs

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Last edited by sterretje; 12-02-2010 at 08:59 AM. Reason: added PS

[Manel Brand]

and therefore lose resolution?

and why is that?

[chrisrussell]

because in stepping back you are further away from the subject.

[Manel Brand]

because in stepping back you are further away from the subject.

so what?

[noblepa]

So will I be taking pictures at 50mm or something else?

Any 50mm lens will capture "full 50mm", whatever that means.

That makes sense.

So I guess my next question is: what 50mm or similar do you recommend which captures full 50mm?

If you are "uninterested in the science", you won't understand. Yes, a 50mm lens on an aps-c sensor gives the same field of view as a 75mm lens on a 35mm film camera or a full-frame dslr. What "bells and whistles" are you talking about?

So essentially when I shoot with my FA 50mm f1.4 on my Pentax K-x - I'm actually shooing with a tighter 75mm equivalent but with all the bells and whistles of a 50mm.

Purchasing a modern 50mm which works with smaller censors will yield a 'wider' photographic view which will actually be 50mm?

50mm is 50mm is 50mm. It is a characteristic of the lens and really has nothing to do with the sensor size. ANY 50mm lens you buy will give you the same field of view as any other 50mm lens.

I learned photography with 35mm cameras. As such, I am used to the fields of view that various focal lengths give me on a 35mm camera. However, when I got my K10D, I learned about crop factor and promptly forgot about it.

"Crop factor" is an entirely artificial term. It has no real use, except to provide some familiarity for those who are used to 35mm. A better approach, IMHO, is to simply learn new reference points. A "normal" focal length is about 33mm. A shorter focal length is a wide-angle lens. A longer focal length is a telephoto lens. 10-12mm is a ultra-wide angle lens. 300-400 is a very long telephoto (yes, I know that there are longer lenses).

I suggest that you forget about crop factors altogether, and just look through your viewfinder and take a few pictures. You will learn what kind of picture that 50mm lens gives you. Now try a 28mm lens and learn what kind of picture that gives you. This is especially true if you do not have a lot of experience with 35mm cameras.

Thinking about crop factors just makes the situation more complicated than it needs to be. The only reason we care about focal lengths at all, is because that gives us an idea of their field of view. If you're shooting an aps-c dslr (which the K-x is), why are you concerned with the field of view on a 35mm camera?

Just my $0.02. My opinion is worth precisely what you paid for it.

[Rondec]

This is something that is difficult to grasp, in particular, because a lot of camera companies have sold crop sensors as a way of lengthening lenses (even Scott Kelby has put it in his books). As others have said, you could get the same image by taking a photo with the a full frame sensor and then cropping it.

Resolution is a completely separate issue. If you take a 12 megapixel full frame sensor and crop it down to APS-C size, you end up with a little bit better than a 6 megapixel image left.

Unless you have shot a lot with film and are used to the way lenses acted on film, I think the comparisons are worthless. It is much better just to shoot a lot with your lenses and learn their depth of field and field of view on the camera you are shooting. Anything else becomes confusing and not helpful.

[Impartial]

So essentially when I shoot with my FA 50mm f1.4 on my Pentax K-x - I'm actually shooing with a tighter 75mm equivalent but with all the bells and whistles of a 50mm.

Purchasing a modern 50mm which works with smaller censors will yield a 'wider' photographic view which will actually be 50mm?

I will give this a try.

When you shoot with your FA 50, you have the optical properties of a 50mm lens. The field of view -- restricted by your digital camera -- is the field of view you would have had with a 75mm lens on a "full-frame" film body.

If you purchase a DA 35 or an FA 35, you will have the optical properties of a 35mm lens. The field of view, though, for your digital camera, will be the field of view you would have had on a 50mm lens (actually 52) on a standard "full-frame" film body.

I would suggest that you (the OP) buy a 35/2.4, since it will give you essentially the same perspective on the world, for APS-C digital, that you had when looking through a 50mm lens in the "full-frame" film world. At least, I find the perspective through a 35mm lens to be familiar and comforting. :-)

[jglover]

because in stepping back you are further away from the subject.

No, resolution is not a function of your distance from the subject, it is a function of your lens' optical properties and your sensor's ability to resolve the detail.

[Lowell Goudge]

sterretje

Nice diagram, too bad your explanation is wrong. You are saying that to get the same FOV as a 50mm on film that yoou need a 75mm lens on digital, but the diagram actually shows that to get the same FOV on film, as a 50mm on digital you need 75mm on film.

THis is why I hate these discussions, too many people offer incorrect or confusing explanations.

the basic sensor comparison is all you need. If you look closely at your 50mm example, yoou crop the middle out of the frame, but the tree is the same hight on both film and digital. That is the point 50mm gives you a specific image magnification at any distance. FOV is wholly dictated by the format of the sensor/film at any focal length , nothing more, nothing less

to everyone else, you will note I have avoided totally telling the OP what to purchase etc, as that is not part of the discussion. the OP and the post was, will my FA50 behave the same way as a digital 50mm. the answer is yes. 50mm is 50mm.