DOF Question

Insp Gadget · Aug 23, 2009

I am looking for some lens info. My wife is looking for a "Portrait" lens. She does wedding photography and likes to blur the background. I assume a portrait lens would be in the 100mm size for our Nikon camera, however I'm not sure this lens will do what she wants.

I am of the opinion that any lens (regardless of maximum aperture) will have the same amount of DOF as long as they are of the same focal length. I was always taught that DOF is caused by the light bending around the leaves of the aperture as you stop down, so would a 100mm 2.0 have the same DOF as a 18-105 4.5, as long as both are open to the maximum?

Hope you can shed some light on this for us!

table1349 · Aug 23, 2009

Insp Gadget said:
I am looking for some lens info. My wife is looking for a "Portrait" lens. She does wedding photography and likes to blur the background. I assume a portrait lens would be in the 100mm size for our Nikon camera, however I'm not sure this lens will do what she wants.

I am of the opinion that any lens (regardless of maximum aperture) will have the same amount of DOF as long as they are of the same focal length. I was always taught that DOF is caused by the light bending around the leaves of the aperture as you stop down, so would a 100mm 2.0 have the same DOF as a 18-105 4.5, as long as both are open to the maximum?

Hope you can shed some light on this for us!

Generally yes, however there are more factors to good Bokeh, that blured back ground, than just the DOF. Any Canon shooter knows that the 50mm f1.8 has ok Bokeh, the 50mm f1.4 has better because of the number of aperture blades used in the f1.4 version.

Here is a good site to review lenses and hear what other have to say about them.

Dwig · Aug 23, 2009

Insp Gadget said:
...My wife is looking for a "Portrait" lens. ... I assume a portrait lens would be in the 100mm size for our Nikon camera,

however I'm not sure this lens will do what she wants.

What the right focal length is depends on your definition of proper framing for a portrait. Its shooting distance that's important for getting the right perspective, not focal length directly. You need to shoot portraits from at least 6ft and possibly as far as 12'. You choose the focal length that gives you the desired framing at that distanct. The right lens for a full length portrait would be quite different from the right lens for a tight head & sholders or face-only shot.

,,,so would a 100mm 2.0 have the same DOF as a 18-105 4.5, as long as both are open to the maximum?...

NO! The zoom set to 100mm would yield the same DOF as the 100mm f/2 if, and only if, they were both set to the same aperture. If set to maximum on each lens, the 100mm f/2.0, set to f/2, would deliver much less DOF than the zoom at 100mm and f/4.5.

There is an aspect of the out of focus portion of the image that is refered to as "bokeh". This term is, unfortunately, very misused on this forum. Most recent posts that I've encountered have used it incorrectly. Reading some of these can lead to some false impressions on DOF when "bokeh" is confusted with DOF in general.

Bokeh, used properly, refers to the qualityof the out of focus portion of the image, not to the quantity or degree of blur. There are a number of optical factors that contribute to a lens having either good or bad bokeh. One is the shape of the aperture (aperture means "hole") created by the iris blades. In general the rounder the shape the better the bokeh. Almost all lenses open the iris blades all the way at maximum aperture leaving a true circular aperture and often the best bokeh (quality of blur, not quantity of blur) that the lens will deliver. Still, there are other very complicated to describe optical characteristics that can heavily influence bokeh.

Insp Gadget · Aug 23, 2009

I'm not sure I understand. I thought DOF is determined by the light bending around the aperture blades, the more the blades cover the lens opening, the more the light has to bend and creates an area with more DOF.

If both lenses are wide open, how does the light bend to create a larger area of DOF in a 4.5 compared to a 2.0?

Derrel · Aug 23, 2009

Depth of field depends quite substantially on the size of the capture format and the lens focal length *relative to* the capture format size. Depth of field also depends *tremendously* on the camera-to-subject distance.Those are the two largest factors that control one's depth of field. The subject-to-background distance is another factor that plays a VERY large part in depth of field,and how greatly the background can be rendered softly out of focus.

The easiest way to get shallow depth of field is to use a larger-format camera, like a FF digital SLR camera, or a medium format film camera, or a 4x5 inch sheet film camera. With each larger format, with its OWN,specific standard lens that measures roughly the diagonal of the capture size, the larger the capture format, the shallower the depth of field. So for "equal" angles of view, the larger the capture format, the more-shallow the depth of field band is. Small-format digital point and shoots and the Kodak Disc format have/had inherently DEEP depth of field. 4x5 sheet film cameras have shallow depth of field. In the modern era, 4/3, APS-C, and FX or FF digital SLR formats are the three most common and most popular formats; for people work, the larger the format, the better, in my opinion.

With any one,specific camera, like a DX format Nikon d-slr, the longer the camera-to-subject distance, the more likely one will get pretty deep depth of field. Because a DX format camera crops off,and does not image the outer image circle, shooting portraits with a 105mm Nikkor means that you must be at a considerable distance from the camera in order to frame, say, a standing person, or a wedding couple. At long distances, with a 105mm lens, the depth of field is somewhat deep. Because the subject is a long ways away from the camera, and the background is even farther from the camera, the 105mm focal length will not really give you the kind of foreground/background isolation on DX that it will on a larger sensor, due to different camera-to-subject distances AND differences in the capture format's size.

If your wife wants a portrait lens that can be used indoors or in normal, outdoor environments, and she wishes to use a DX format Nikon, the 85mm f/1.8 AF-D Nikkor or the 85mm f/1.4 AF-D NIkkor are the two most handy lenses; they have wide apertures, focus well,are relatively small and relatively light, and will give a working distance that is "acceptable" (but nowhere near idea) on DX format--even though she will need to be around 32 feet way in order to photograph a 6 foot tall man and his bride and allow enough head space and foot space for proper presentation, or to allow an 8x10 crop; if she had a FF Nikon, like a D700, she could use the same 85mm lens and get shallower depth of field, better foreground/background subject isolation, AND the SAME field of view height and width from only 20 feet away from the people being photographed.

Bottom line: if one is really serious about people work and portraiture, one needs to understand that the DX format is over twice as small as FX, and that the FX format will give the photographer many more lens options, as well as much better background control and more-appropriate real-world shooting distances. If she wants a portrait lens for DX, think 85mm more so than 105mm.

Josh66 · Aug 23, 2009

2.0 is "more wide open" than 4.5...

Just because they are both wide open does not mean they are the same.

Insp Gadget · Aug 23, 2009

Derrel said:
Depth of field depends quite substantially on the size of the capture format and the lens focal length *relative to* the capture format size. Depth of field also depends *tremendously* on the camera-to-subject distance.Those are the two largest factors that control one's depth of field. The subject-to-background distance is another factor that plays a VERY large part in depth of field,and how greatly the background can be rendered softly out of focus.

The easiest way to get shallow depth of field is to use a larger-format camera, like a FF digital SLR camera, or a medium format film camera, or a 4x5 inch sheet film camera. With each larger format, with its OWN,specific standard lens that measures roughly the diagonal of the capture size, the larger the capture format, the shallower the depth of field. So for "equal" angles of view, the larger the capture format, the more-shallow the depth of field band is. Small-format digital point and shoots and the Kodak Disc format have/had inherently DEEP depth of field. 4x5 sheet film cameras have shallow depth of field. In the modern era, 4/3, APS-C, and FX or FF digital SLR formats are the three most common and most popular formats; for people work, the larger the format, the better, in my opinion.

With any one,specific camera, like a DX format Nikon d-slr, the longer the camera-to-subject distance, the more likely one will get pretty deep depth of field. Because a DX format camera crops off,and does not image the outer image circle, shooting portraits with a 105mm Nikkor means that you must be at a considerable distance from the camera in order to frame, say, a standing person, or a wedding couple. At long distances, with a 105mm lens, the depth of field is somewhat deep. Because the subject is a long ways away from the camera, and the background is even farther from the camera, the 105mm focal length will not really give you the kind of foreground/background isolation on DX that it will on a larger sensor, due to different camera-to-subject distances AND differences in the capture format's size.

If your wife wants a portrait lens that can be used indoors or in normal, outdoor environments, and she wishes to use a DX format Nikon, the 85mm f/1.8 AF-D Nikkor or the 85mm f/1.4 AF-D NIkkor are the two most handy lenses; they have wide apertures, focus well,are relatively small and relatively light, and will give a working distance that is "acceptable" (but nowhere near idea) on DX format--even though she will need to be around 32 feet way in order to photograph a 6 foot tall man and his bride and allow enough head space and foot space for proper presentation, or to allow an 8x10 crop; if she had a FF Nikon, like a D700, she could use the same 85mm lens and get shallower depth of field, better foreground/background subject isolation, AND the SAME field of view height and width from only 20 feet away from the people being photographed.

Bottom line: if one is really serious about people work and portraiture, one needs to understand that the DX format is over twice as small as FX, and that the FX format will give the photographer many more lens options, as well as much better background control and more-appropriate real-world shooting distances. If she wants a portrait lens for DX, think 85mm more so than 105mm.

Thanks Derrel! Very informative.

Perhaps I missed it in all this, but if all parameters stayed the same (subject distance, camera etc) would the f1.4 give less DOF than the f1.8 assuming they were both wide open?

Dwig · Aug 23, 2009

Insp Gadget said:
...I thought DOF is determined by the light bending around the aperture blades,...

Its the glass that bends the light. DOF is largely controlled by the angle of the two sides of the cone of light that it projected on the sensor/film.

Think of it this way, a column of nearly parallel light rays coming from an extremely distant extremely small subject strike the lens. The lens bends the rays so that they all meet at one point. This focuses an image of the light source at the imge plane. The light leaving the lens is cone shaped. If this column of nearly parallel light rays is coming from a closer subject, equally point like, they are less parallel and the cone created by the lens will be taller and thus come to a point behind the sensor/film. The sensor/film effectively truncates the cone leaving the image that is a small circle instead of a point.

Now, if the lens aperture is large (f/2) these cones have a large base. If the aperture is small (f/4, f/8, ...) the cones have smaller bases. The truncated cone will have a proportionally smaller circle at the image plane when its base is smaller. The closer subject can be closer, when the aperture is smaller, before its blur becomes noticable, hence greater DOF.

There is a effect call diffraction that occurs as light passes an opaque edge. This causes the light rays that graze the edge to bend. The doesn't not actually affect DOF. What it does is to set a resolution limit that can be achieved by a lens. The smaller the f/stop the lower the maximum possible resolution. This is why stopping a lens down too far in an effort to gain more DOF can actually result in a picture that is less sharp. Stopping down a little can, and usually does, reduce the effect of various optical flaws and compromises and improves the image quality. But at some point the quality loss from increasing diffraction out weighs the improvement. The general result is that lenses tend to perform their best 2-3 stops from maximum aperture.

marp · Aug 23, 2009

This page Online Depth of Field Calculator helped me, when I was trying to understand how various parameters affect the DoF.

DScience · Aug 23, 2009

To answer your question, NO focal length is not the only thing that affects DOF. A 100mm lens at 4.5 will have a greater DOF than a 100mm 2.8, thus the 2.8 will give you a more blurred background that your wife is looking for.

I would recommend either a 50mm 1.8 or 1.4, or the 85mm 1.4 or 1.8.