Originally posted by WMBP Can someone explain - or point me to an explanation of - the difference between an N mm lens used on a digital SLR and the same lens (or at least a lens of the same focal length) used on a film SLR? I gather that an N mm telephoto lens used with a digital SLR gets me "closer" (provides greater magnification or a smaller field of view) than the same lens used on a conventional film SLR, but I'm not sure why. I would also like to know if this is a constant, that is, does the Tamron 18-200mm zoom I use with my Pentax provide the same range as the same lens used with a Canon or Nikon digital SLR?
And is there a standard for lens focal length now and is it the digital value or the film value? In other words, if somebody says a photo was shot with a 300mm lens, do I have to know what kind of camera was being used in order to know what that focal length means?
If this question doesn't quite make sense, well, that would indicate that my confusion is deeper than I know, so I'd be grateful if someone would slap me and straighten me out. Thanks,
Will
I'm not sure I can explain anything. But you can have my point of view on it:
A lens is an isolated optical system. Let's think of a great lens; the SMCP FA31/1.8... Focal length (FL) is 31mm. That's it. The FL doesn't change when moving the lens from an Pentax film SLR (fSLR) camera to a Pentax digital SLR (dSLR). The FL remains the same when unmounting the lens, and when holding it pointing up side down in your hand.
The lens give you a certian field of view (FOV). Think of the camera seen from above. Imagine rays of light entering through the lens hitting the film in the fSLR: you get a certain FOV measured over the 24x36mm film surface. It's most common to measure diagonally over the picture. As we are looking at the system from above we measure from the left to the right edge of the picture. Ok?
Now we know Pentax (unfortunately in my opinion) use a sensor that is smaller than 24x36mm. It is just 23.5 mm wide. In the case of the fSLR the distance from the center to either the left or right edge of the picture is 18mm. In the dSLR case it is a little less than 12mm from the center of the sensor to any of the side edges. Meaning?
Everything that was captured with the fSLR camera doesn't fit on the sensor. (Remember that the distance between the lens and film/sensor is the same in both cases.) Parts of the picture is cropped away - hence the expressions "crop factor" and "cropped sensor".
So, the FOV has changed while everything else is the same. The FL remains the same and the magnification factor (how much the lens magnifies the subject) are the same. The only thing that changed is the FOV.
So, the lens when used with a "cropped sensor" doesn't bring you closer or increase the magnification. The impression you get while looking at the subject (a distant bird, perhaps) is that the bird fills more of the picture compared to the same situation/location with a fSLR camera. This is important to understand as it is the key to understand the rest of the effects following to this relatively simple point:
Side effects:
* You "get closer" with regards to take advantage of the available sensor or film size. Bird photographer like this.
* You loose some surroundings. Wide angle freaks dislikes this.
* The depth of focus (DOF) has changed a little (I won't go deeper into this subject as it should only make us look like a crcle of confusion right now)
* You get less distorsion, less vignetting and better edge resoultion thanks to the fact that we use only the central part of what the lens is made to be able to cover. It's always the edges that loose out when you push a lens to it's limits (test it). This can be of great or little advantage depending of what lens you are using.
Hmm. Good enough for a start I think. I surely forgot some basic things but I'm also sure other members will fill in.
Two links:
DPR editorial on the subject:
Focal Length Multiplier: Optical: Glossary: Learn: Digital Photography Review
DOF and calculating the same:
Hyperfocal Distance and Depth of Field Calculator - DOFMaster
I'm aware there are better explanations but this is the way I like to think about it.
Some standard lenses with a fSLR (dSLR 1.5 crop) camera:
80-90mm (50-60mm): considered a short tele, good for head an shoulder portraits giving nice proportions to facial details
45-50mm (28-35mm): The standard lens since ages
35mm (22-24mm): moderate wide, good for street shooting and indoor use
20-21mm (14mm): extreme wide for perspective effects and covering a lot
regards,
EDIT: Lol, yes I type slowly...