Sensor size, vs, DOF & aperature ratings.

Sorry to bother all of you, but I always wanted to know, How does

sensor size effect the DOF and Aperature of your pictures? Is it the

same as 35mm?

 

Thanks,

Dan

Aperture remains the same (f/2.8 remains f/2.8) depth of field remains the same for

the focal length of the lens. BUT the angle of view changes. For instance, consider a

50mm lens on a Nikon digital SLR. Due to the smaller sensor size the angle of view

captured is the same as what you would get using a 75mm lens on a Nikon film

camera, but as I said before, the depth of field remains the same as a 50mm lens and

not that of a 75mm lens. This is what confuses so many people: with the change in

angle of view captured they expect the depth of field, at a given f-stop to change to

that of a longer focal length lens.

I am responding to you because this suject is new to me and I need the mental exercise figuring it out for myself. I hope I get it right. If not we'll both be set straight by some other contributor to this forum. Sensor size in the P&S digital cameras is smaller than 35mm. So the lens magnification also differs. My 5.8mm in the Nikon 5400 is equivelent of about 28mm in 35 format. The aperture at f2.8 however is still f2.8 in relation to the 5.8mm (magnification?) lens. So at 5.8mm (or equivelent 28mm, nice wide angle of view) everything is in focus (bummer!). The DOF is anything but shallow.

I can get close to a subject and zoom out to achieve shallower dof with the 5400. For me and at this point, I live with the sacrifice because the DSLRs currently available are unappealing. I far prefer my rangefinder film cameras that allow me to shoot portraits beautifully. Even at wide angle (and f1.4 or f2).....but I am ranting.

Have a look at the photo.net article: <a href="http://www.photo.net/learn/optics/dofdigital/">Depth of Field and Digital</a>

Angle of view changes and so does your distance from the subject. The size of the aperature also changes as it is a realitive thing (called f stop). And the smaller the physical hole relative to the size of the image (sensor or film) has a direct comparison on depth of field. Soooooo. Smaller sensors have greater depth of field relative to field of view. Get an old Kodak 2 meg and be amazed. EVERYTHING is in focus. 4x5 cameras have a huge "sensor" and a huge aperature to let in enough light to maintain the same relative exposure (f stop) - and they need to shoot at f64 and use tilts, swings, etc. With same field of view and same f stop, smaller aperatures and smaller "sensors" produce greater depth of field. Hence, everything else being proportional, smaller sensors produce greater depth of field than 35mm. This is called the "circle of confusion" in scientific terms - AND IT SURE IS.

"The size of the aperature also changes as it is a realitive thing (called f stop). And the

smaller the physical hole relative to the size of the image (sensor or film) has a direct

comparison on depth of field. "

 

no it doesn't.

 

"Angle of view changes"

 

 

 

yes.

 

" and so does your distance from the subject."

 

only if you move.

 

"Get an old Kodak 2 meg and be amazed. EVERYTHING is in focus. 4x5 cameras have

a huge "sensor" and a huge aperature to let in enough light to maintain the same

relative exposure (f stop) - and they need to shoot at f64."

 

well the two formats use radically different focal length lenses to capture the same

angle of view from the same position. A "normal focal length lens for 4x5 is 150mm

(rounded off), a lens for the Kodak 2Mp camera is probably something like 8 mm. An

8mm lens at f/5.6 is going to give you radically different depth of field than a 150mm

at f/5.6 when used from the same position. Format is irrelevant until you consider the

angle of view captured by the different sizes of media. f/8 on a 50mm lems remains

f/8 on a 50mm lens no matter what size media is used: the same amount of light is

transmitted One thing that does change perceptuallywith different size capture

media is perception of "sharpness" ( resolution vs. diffraction). The smaller the format

the more critical the resolution of the lens is. Basically because that smaller image

must be magnified (enlarged) more to get to a given enlargement size.

 

 

An f/5.6 aperture lens on a 150mm lenss is larger than f/5.6 on a 50mm lens

because the nodal point is a lot further fronm the media plane.

 

 

By the way I actually shoot a pretty fair amount of 4x5 and I very, very very rarely

shoot at f/45 much less at f/64.

<<""The size of the aperature also changes as it is a realitive thing (called f stop). And the smaller the physical hole relative to the size of the image (sensor or film) has a direct comparison on depth of field. "

 

no it doesn't.">>

 

I would quibble with Ellis's wording - a 100mm f/4 lens has a maximum 'hole' 25mm in diameter - period. Regardless of whether it's a wide-angle for a 4x5, or a "normal" on a 6x7, or a telephoto/long tele on a 35mm or a digital. "f/4" means "focal length (f) divided by (/) the opening diameter".

 

A digital sensor (assuming it isn't a full-frame one) crops your image - but the basic underlying image the lens projects otherwise remains unchanged. Use a 35mm lens on a partial-frame DSLR, and you will get the cropping (field of view) of a 50, and the DOF of a 35.

 

There is one other factor - mostly insignificant in the small change in sensor size from 35mm to DSLR, but somewhat significant when talking about the really small digicam sensors. Enlargement factor.

 

A "normal" 10mm lens on a digicam (say, a Canon G5) will have the DOF of a 10mm lens - ON THE SENSOR. But since you must enlarge that image 5x more than a full-frame 35mm-film image, some of the circles of confusion will be enlarged to the point where they no longer look like sharp points, but start to be visible blur cicles.

 

In other words - the DOF of a 10mm image from a G5, enlarged 40x to make an 8" x 10" print, will look like a crop from a 10mm-lens image on 35mm film (ignoring the grain effects), enlarged 40x to make a 40" x 60" print. So there will be somewhat more blur visible than in an 8 x 10 print from the full-frame 35mm "10mm-lens" image. But substantially less blur than from a "normal" 50mm lens on film.

 

Same applies for 4x5 vs. 35mm - the 4x5 image needs only a 2x enlargement to reach 8x10 - so some things that might appear slightly OOF in an 8x enlargement from a shot made with a 80mm lens on 35mm film will still appear sharp in a print from a 4x5 neg shot with the same 80mm lens at the same aperture.

 

As to aperture ratings - a given f/stop ("f/2" or whatever) delivers the same light intensity to the film/sensor surface in all cases.

 

A 10mm f/2 lens delivers the same brightness per square mm as a 50mm f/2 lens. But it has to cover a much smaller area (fewer square mm), so it can use a smaller absolute opening (10mm/2 = 5mm vs. 50mm/2 = 25mm).

 

Which is a long way of saying - if the meter calls for 1/250th sec. @ f/2 with an ISO100-speed film/sensor - you'll get the same exposure @ 1/250th with a 10mm f/2 lens on a chip, or a 50mm f/2 lens on 35mm film, or a 100mm f/2 lens on 6x9 film.

Andy,

 

"As to aperture ratings - a given f/stop ("f/2" or whatever) delivers the same light intensity to the film/sensor surface in all cases. "

 

Actually, this is the behavior of T stops, but not F stops, although for still photography the difference is of little consequence.