 Allan |
depth of field I have a 28mm-200mm 3.8/5.6 tamron lens and also a 50mm canon 1.8 lens. If I set the zoom lens to 50mm and have both lens set at 5.6, I would imagine that the size of the aperture at this setting on the zoom lens would be larger due to its construction(extra elements). If this is the case,does that mean at the above settings I will have less depth of field on the zoom lens. Thanks Allan
|
|
|
|
||
|
Ken Pang |
No. I don't fully understand how DoF works, but regardless of which lens does it, 50mm @ f/5.6 will always have the same DoF. Ken.
|
|
|
|
||
|
Jeff S. Kennedy |
Ken's right. At a given focal length (angle of view) the DOF will be the same at a given aperture. The lens construction doesn't matter since f-stops are merely ratios of aperture diameter to lens focal length. That being said, the 50mm lens may appear to have more critical sharpness because it is a prime lens but the area in focus remains the same.
|
|
|
|
||
|
Allan |
thanks Ken and Jeff. Jeff with respect to the f-stop being a ratio of aperture diameter to lens focal length,would it be fair to say with both lens set at 50 mm, the zoom lens's metering system may choose a slower shutter speed to maintain the same exposure as the fixed lens.
|
|
|
|
||
|
Ken Pang |
No, it wouldn't. If you have the same angle of view (determined by focal length), and the same size aperture - then it would stand to reason that the same amount of light reaches the film plane. So... If you have both lenses set at 50mm f/5.6, then both lenses have the "hole" at 8.9mm (approx) Now consider both lenses are looking at the same picture (since they are both 50mm, they will have the same angle of view). The only thing that could possibly change the light intensity coming out the back side of the hole, is the light intensity coming in to the front of the hole. If we consider that glass is almost lossless transmission of light, then exactly the same amount of light is passing through exactly the same size hole. Which means, exactly the same amount of light comes out the side (on to your film plane) It doesn't really matter if glass on one of them moves, is bigger or whatever. Same image on same size hole = same brightness on the other side.
|
|
|
|
||
|
Jeff S. Kennedy |
Ken's right. Since you are dealing with the same ratio you are talking about the same quantity of light hitting the film. The shutter speed would be the same regardless of whether you are shooting with a zoom or a prime. That's why f-stops are based on ratios so that they translate between lenses of different focal lengths. If you meter the scene with a 50mm lens and get an exposure of f8 @ 1/125 and meter the scene with a 100mm lens you will still get f8 @ 1/125.
|
|
|
|
||
|
John A. Lind |
You're mostly right about DoF, at least in the end result. There is a common misconception about DoF being based purely on angle of view though (focal length). More correctly, it is based on three factors: film size, magnification (of the subject on film), and lens aperture. Subject magnification is a combination of focal length and distance from the subject (and as a result the focus distance). If you switch from a 50mm to a 100mm lens, use the same aperture setting, and move back twice as far from the subject (setting focus distance twice as far) thinking you will have the same subject size with a shallower DoF, you won't. It will be the same. Why? Subject magnification remains the same on the film. The effect of decreasing the DoF with increasing the focal length occurs at the same rate as increasing the focus distance (subject distance) increases the DoF. The two cancel each other out *exactly* if subject magnification is kept constant (on film; same as in viewfinder). In other words, the DoF of a 50mm lens at f/5.6 focused at 20 feet is exactly the same as for a 100mm lens at f/5.6 focused at 40 feet, or a 150mm lens at f/5.6 focused at 60 feet. Where does this lead to? If you want the subject to be a specific size in the image (on film) you cannot change DoF by changing focal length (and moving closer or farther). For the same subject magnification, only a lens aperture change will change the DoF. Some may note that apparent DoF will seem shallower if larger prints are made from the same piece of film. True only if they are viewed at the same distance, but not if the larger print is viewed from a greater distance in keeping with the size increase compared to a smaller print. -- John
|
|
|
|
||
|
Jaymes R. Stuart |
All of the above is true, but John touched on an important point when he mentioned film size. Aperture numbers are relative and not absolute, and it's not the aperture number which affects DOF; it's the actual physical size of the hole that's letting the light in, and that varies with film size. Example: at f2.8, a point-and-shoot digital with a leetle tiny CCD will have a whole lot more DOF than a 35mm camera at the same aperture, because the hole is smaller. Depending on the CCD size, it's like shooting 35mm at f8 or f11. Just ask a digicam owner who's trying to get "bokeh"...there ain't none! So, when Ken says "50mm @ f/5.6 will always have the same DoF"...add the caveat "unless you change the film size".
|
|
|
|
||
|
John A. Lind |
Jaymes, Ken's right, but it isn't because the aperture diameter is smaller. f/5.6 has the same effective aperture diameter on a 50mm focal length lens, regardless of film format (size) that the focal length is used with. The f-number is the lens focal length divided by the effective aperture diameter. A 50mm focal length is a long lens (telephoto) for APS, a standard lens for 35mm format, and a short lens (wide angle) for medium format (e.g. 645). An 80mm lens on a 645 camera has about the same angle of view as a 50mm lens on a 35mm camera. Its DoF is shallower for the same focus distance and aperture compared to a 50mm lens on a 35mm camera because it's an 80mm lens. However, it's also deeper than using an 80mm lens on a 35mm camera at the same focus distance and aperture. It's because the DoF near and far limits are determined by magnification (focal length and focus distance), lens aperture and _film_size_ (read on to see how this affects it). I left out how film size affects DoF in my previous reply because it's more complex. DoF has one other somewhat "fuzzy" component called the maximum acceptable "circle of confusion" (CoC). When an infinitessimally small pinpoint is at any distance other than exactly at the critical focus distance, it is out of focus and forms a "circle of confusion" on the film plane. We normally view photographs with the "unaided" eye (no magnification) at a viewing distance that fills the primary field of vision. The human eye has finite acuity. To be perceived as out of focus, the CoC must be large enough for the human eye to detect it's no longer an infinitessimally small pinpoint, but a circle with finite diameter. The acuity of the average human eye defines near and far limits for DoF. Because various sizes of prints are made and tend to be viewed at different distances based on print size, the value used for the maximum CoC is set by film size, and it the maximum diameter CoC _on_the_film_ not in the print (or projection if it's a slide). Values used for maximum CoC on 35mm size film size range from 0.025mm to 0.030mm (on the film). A larger piece of film is not enlarged as much to make the same size print. Therefore a larger CoC is used; for 645 medium format, values used range from .050mm to .060mm (on the film). This is why I called it a "fuzzy" component. The exact maximum CoC value that should be used depends on who you ask. It's based on human perception, the "average" human eye and an "average" viewing distance. The rest of DoF (near and far limits) is lens and film geometry to calculate how much closer and farther than the critical focus distance a pinpoint must be to create a circle the size of the maximum CoC on the piece of film. Does the growth allowed in maximum CoC by moving to a larger film size fully compensate for the growth in focal length to keep the same angle of view? NO! In the DoF equations, the focal length is squared. The maximum CoC is not. That's why the DoF shrinks some with increase in film size, and grows some with decrease in film size. As an aside: Another aside: -- John
|
|
|
|
||
|
This old forum is now archived. Use improved Forum here
Report this Thread |
||