[StudioJ]

So I haven't done major investigation into how the whole technology of the digital sensors work, I know they are smaller and thus there is some form of cropping or multiplier as they call it.

What I want to know though is when you have a lens that isn't as wide anymore or a zoom that is longer does it keep the characteristics of the focal length?

What I mean is using a 50mm going to give the same effect on body features as the equivalent 80 (85mm) lens? Or at 135mm being the same as 200mm having a flattening affect?

I've read of using a 50mm lens giving somewhat exaggerated upper body and heads and noticed that on a few of my shots and wondered if it were perhaps bulbous people or whether the lens truly does take on the features of a longer lens?

[shuttertalk]

I believe that the perspective is still the same, it's just that the image is effectively cropped with the multiplier. It's as if you took a full frame image and then cropped the centre portion out.

I think with certain lenses such as extreme wide angle or fisheyes, most of the lens characteristics occur towards the edges of the frame, so you might lose some of the effect.

[StudioJ]

Hmm that makes a lot of sense as far as wide angles go, thankfully we lose most of our detail and have the unwanted effects appearing on the edges with most lenses.

I wonder if anyone has done anything on this, what bought it to my attention was the unusual proportions on a couple of girls heads. A lot of folks run around touting the glory of using a 50mm lens to give the same effect as a portrait lens and I was pretty sure it was wrong.

[slejhamer]

There is much debate on this issue, and people argue the many different sides. This is my understanding:

First, to the extent that focal length influences DOF, a 50mm lens is a 50mm lens.

Second, if you crop an image taken with a 50mm lens on a full-frame (35mm) camera and compare it to an image taken on a digital SLR with a field-of-view crop/multiplier, you should find that perspective is exactly the same provided that distance to the subject remained the same. Again, a 50mm lens performs like a 50mm lens, just with a smaller FOV, as ST stated above.

However, if you MOVE THE SMALLER-SENSORED CAMERA BACK so that the subject is framed the same as it was in the full-frame pic, then you fill find that the PERSPECTIVE is now that of an 80-85mm lens used on the full-frame camera at that same longer distance. Distance to subject is the key factor influencing perspective distortion; wide lenses seem to increase perspective distortion because you must move closer to the subject in order to achieve the same framing. Thus, for practical purposes, the 50mm lens does indeed perform like the 80-85mm lens with respect to perspective and FOV, but again not with respect to DOF.

Clear as mud now?

[shuttertalk]

Wow good point, Mitch. I guess if you move or zoom in, then it changes the whole story...

[Cailean]

OK, well, I'm going to semi-hijack this thread and ask about optical zoom vs. "35mm equiv."

My Lumix has 12x optical zoom which claims to be equiv. to 420mm. Fair enough. The focal length of the lens, however, is obviously NOT 420mm. The max f-length on my cam is 72mm.

So, my question (I guess to slej since he seems to have a clue about these things) is:
Does my lens behave like a 72mm or a 420mm lens? (assuming the zoom is maxed)

[StudioJ]

Ok Mitch so is the cropping of a 50mm lens going to make the subject need to be at the distance of an 80-85mm to fit into the frame? Or is that just moving the subject back and not filling the frame still?

[slejhamer]

Does my lens behave like a 72mm or a 420mm lens? (assuming the zoom is maxed)

No, and yes.

First, it sounds like you may be mixing terminology. 12x optical zoom is just the relative change in focal length from your widest lens setting to the longest. It has nothing to do with the "multiplier" or "crop" factor, which is a function of sensor size compared to the size of a frame of 35mm film. So if your actual focal length is 72mm and that gives you a 35mm-equivalent of 420mm, then your crop factor is about 5.8x. At any actual focal length f, you can multiply f by this factor to get the full-frame equivalent.

So, in terms of field-of-view, yes, you are seeing more-or-less the same image you would would see in an image taken with a 420mm lens on a full-frame (35mm) camera. I say more-or-less because most digicams use a 4:3 aspect ratio, while a 35mm camera more typically uses a 3:2 aspect ratio.

But in terms of depth-of-field, they are definitely not the same. In fact, at any f/stop, you can use the same multiplier to estimate the full-frame equivalent aperture and thus DOF given by your lens. Among other factors, DOF is a function of actual focal length, which in your case is quite small (smaller f = greater DOF, all else constant,) and circles of confusion, which on a small-sensor digicam are very small and produce deep DOF. So, for DOF, your lens does not behave like a full-frame 420mm lens, nor does it behave like a full-frame 72mm lens!

[slejhamer]

Ok Mitch so is the cropping of a 50mm lens going to make the subject need to be at the distance of an 80-85mm to fit into the frame? Or is that just moving the subject back and not filling the frame still?

Well you probably want to move the camera back rather than moving the subject, but yes, with for example a Canon 20D's 1.6x factor, you would move the camera with 50mm lens back to a position that is the same as where you would be with an 85mm lens on a full-frame camera. Thus FOV (framing) and perspective are the same. That's why people will tell you that the camera's crop factor necessitates more working space when working with lenses that previously allowed you to stay fairly close to the subject.

_______
Footnote: The FOV comparison is accurate ONLY WHEN FOCUS IS SET TO INFINITY. Why? I don't know. But I learned about this recently when trying to compare two different zoom lenses with camera on tripod and subject at a constant distance and both lenses set to 70mm. At any focus distance less than infinity, the longer lens will have a smaller FOV than the shorter lens. In my case, an image that filled the frame on a 70-200mm lens set to 70mm did not fill the frame with a 28-75mm lens also set to 70mm, unless focus distance was at infinity. I think it has something to do with the changing position of the focal elements as focus is adjusted (the elements actually move back and forth within the lens.)

[Cailean]

Thanks, Slej... it is quite a challenge to learn the terminology of photography in such a transitional time (film->digi)...

As far as cropping & magnification, etc. goes, I found this article which may help make sense of what Slej has already said...

[shuttertalk]

Good find! Thanks Colin...

[adam]

Yes, a 50mm lens is still a 50mm lens, as I understand (which isn't much since I haven't used FF slr) it's just that your field of view is reduced.

It'll be nice (and quite expensive) for a DSLR with FF.

What are the advantages of this cropping?
That we can use EF-s lenses (which aren't that much cheaper than others)?

[shuttertalk]

Advantage touted by manufacturers is that your telephoto end gets a "longer" reach...

However, the drawback is that the wide angle end has a much narrower field of view.

The EF-s lenses are supposed to be better and cheaper to produce because they are created specifically for the smaller frame sensors, but *shrug*...

[matthew saville]

The main thing is that your viewfinder is smaller. The size of the viewfinder reflects the size of the sensor or film, obviously, unless you' have a rangefinder. There is no way to "magnify" what you see through the single lens reflex system, as far as I know...


What most people don't think about is Depth of Field. Think about it. If you're shooting with a 200mm lens and have a 1.5 conversion factor, then they say "you're shooting with a 300mm lens", right? Almost. People get excited because they're now able to shoot at 300mm with the faster apeture of a 200mm lens. This is true, but what you're REALLY shooting with is a 200mm lens, not a 300mm. There IS a very noticeable DOF difference between a 200mm lens with 2.8 apeture and a 300mm lens with 2.8 apeture. It all stems from the physics that allows one to shoot a 17mm lens at 1/20 of a second handheld, and get good results. If you shoot at 17mm and open your apeture all the way up to lets say f/ 2.0, even if you focus two feet in front of your camera your infinity background will still be identifialble, instead of thrown entirely out of focus. If you were shooting with a 300mm lens at f/ 2.0, you'd get a plane of focus about as thin as a hair and then everything further than six inches from that plane is toing to be entirely blurry. Even if you cllose down to f/ 32, if you're focused at minimum, infinity is going to be entirely blurry.

So the whole point is that when you shoot with 200mm and think you're shooting at 300mm, your DOF isn't going to be as versatile. It's hardly a reason for someone who just shoots pictures of their kids to worry, but when you're trying to do semi macro, the wider angle DOF can get annoying if you don't want it.

All in all I think a crop factor is good if you realize that telephoto lenses are far more expensive than wide angle lenses. But man, I absolutely love the spacious and bright viewfinder in my 35mm body! That's what I'll miss the most...

Cheers!

-matt-