Effect of Adapters & added equipment on depth of field

[Paul Bowen]

I am confused by the effect on depth of field caused by the addition of such things as extension tubes, teleconverters, ND filters, etc. When the aperture of a lens is set at say f8, then there is a related depth of field. If a 1.4X teleconverter is added, there is a one stop light loss. Does that mean the the depth of field is now that of a f11 lens even though the aperture has not changed? If a 2 stop neutral density filter is added, does that mean the depth of field is the same as f16 even though the aperture is still set at f8? If a extension tube is added to change the focal distance and increase magnification, does the depth of field change if the aperture remains at f8?

Thanks for your help

[John Morton]

The aperture settings of any lens are in fact a ratio between the size of the opening and the length of the lens: the length of the lens divided by the size of the aperture opening. That's why the aperture number gets larger as the actual aperture gets smaller: the smaller the opening is, the more times it can be divided into the length of the lens.

So, if you add anything that increases the length of the lens (an extension tube, for instance) then the effective aperture will be smaller, because the actual aperture will now divide into the length of the lens more times.

That means an increase in the depth of field, right? Well, not so fast; there is something else to consider here!

You've increased the size of the object your are photographing as it falls on your camera sensor, resulting in it being magnified; BUT you've also changed the length of the lens, so you are going to need to refocus the lens. In effect, the object you are photographing is now much closer; and as we all know, the closer a lens is to what it is focused upon, the shallower the depth-of-field available.

In my experience, the fact that objects are now "closer than they appear" greatly outweighs any minor depth-of-field advantage that may or may not be obtained by an apparently smaller aperture.

There are equations to determine exactly what is happening with any particular lens augmentation combination; they were very commonly used in the days of manual focus lenses and film. I have pages on that in old Kodak manuals but, not ready to hand.

Another way to think of it: I've seen very compelling demonstrations which illustrate why wide angle lenses do not, in fact give greater depth-of-field than telephoto lenses (contrary to popular mythology). The demonstration runs thus: if you take a photo of a scene with a wide angle lens and a distinct object in the foreground, and then from the same spot with a telephoto lens, the wide angle photo will have a much greater depth-of-field; BUT, if you move back with the telephoto so that the scene it captures encompasses the same image area (with the aforementioned foreground object being the same size in both wide angle and telephoto images) as the wide angle shot, the depth of field will be the same (at the same aperture). The difference will be in point-of-view perspective, which will make objects close to the wide angle lens appear relatively larger in the photograph than they are in the telephoto shot.

[Paul Bowen]

So how about neutral density filters or other devices that don't change length of lens or size of subject on sensor? I have a 500mm reflex lens with fixed aperture of f8. It also has built in neutral density filters that can be rotated into place, in which case, the rotating ring (that moves the filters into and out of the focus path) is marked f11 and f16. Is that nomenclature wrong as far as depth of field goes, since the lens length has not changed nor has the magnification?

[Manfred M]

No, ND filters have no effect on DoF per se. Optically, you are effectively putting a set of sunglasses in front of your lens, so this does nothing to change either the focal length or aperture.

ND filters tend to be used for one of two purposes (assuming no change in ISO settings); the first if to increase the exposure time so that movement is blurred. The other case is to enable you to shoot at a larger aperture than one would use without the filter (at the same shutter speed) and this reduces the DoF in the image.

[Paul Bowen]

The instruction sheet that came with the reflex lens mention says in part:

"It is recommended that a tripod be used with this lens................. Using the installed neutral density filters will also insist with focusing. The depth of field at f8 is quite narrow. By using the neutral density filters, the effective aperture is reduced to either f11 or f16, which increases the apparent depth of field and making focus easier to obtain. While, the shutter speed will also increase, this is countered by mounting on a stable tripod, allowing for longer exposures..........."

These instruction seem to be at odds with your reply. You would think that designers of lenses would provide accurate instructions, but?

[pnodrog]

As written the instructions are wrong.

Shutter speed will decrease as aperture value(eg f8 going to f11) is increased. ( Hence the tripod recommendation) Aperture value (or shutter speed) should decrease when a ND filter is used.

I think the translator was not a photographer.

[DanK]

DOF is a function of aperture, not of the amount of light passing through the lens. An ND filter is relevant for focusing not because of DOF, but because it is harder to focus in dimmer light.

[ajohnw]

The instruction sheet that came with the reflex lens mention says in part:

"It is recommended that a tripod be used with this lens................. Using the installed neutral density filters will also insist with focusing. The depth of field at f8 is quite narrow. By using the neutral density filters, the effective aperture is reduced to either f11 or f16, which increases the apparent depth of field and making focus easier to obtain. While, the shutter speed will also increase, this is countered by mounting on a stable tripod, allowing for longer exposures..........."

These instruction seem to be at odds with your reply. You would think that designers of lenses would provide accurate instructions, but?

What make of lens is it. The original reason for fitting neutral density filters to mirror lenses relates to taking shots in bright light when cameras tended to have much lower maximum shutter speeds than they do these days.

John
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[Manfred M]

It does sound like there is a problem with the lens instructions, as these do not really seem to make any sense.

The only lenses that I can think of that "require" ND filters are catadioptric lenses (a.k.a "mirror lenses"). Is this the case with your lens? A maximum aperture of f/8 can actually be typical in this design.

[ajohnw]

It does sound like there is a problem with the lens instructions, as these do not really seem to make any sense.

The only lenses that I can think of that "require" ND filters are catadioptric lenses (a.k.a "mirror lenses"). Is this the case with your lens? A maximum aperture of f/8 can actually be typical in this design.

I thought i saw the term reflex which is often use to describe them. In real terms if said ND filters had differing sized holes in them they could in principle be used to alter the actual F ratio if put in the correct place. I'm not aware of any one who has done this so doubt if this is the case. I also vaguely recall some company or the other mentioning increased depth of field when an ND filter is added which is complete and utter rubbish. The 2ndry mirror could be used to actually alter the F ratio but I don't think any one has done this. The higher the magnification it gives the better really from the obstruction effects point of view. This is why the larger diameter ones tend to give better results if made correctly even though all are F8. From what I have seen of faster ones I wouldn't touch one with a barge pole. The main mirror isn't big enough and lower magnifications from the 2ndry mirror increases it's diameter. Sad really as they could be a lot better..

John
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[Polar01]

I think it maybe quite simple, as the lens is f/8, then using one ND filter it states F/11 (one stop) if using the other ND filter f/16 (2 stops). This reminds you to rework out the exposure time that you got at f/8 to now equal either f/11 or f/16 the actual f-stop does not change as it is fixed and as such the DOF also does not change.

Cheers: Allan

Below is a link on the Nikon 500mm f/8 Reflex

http://www.kenrockwell.com/nikon/500mm-f8-n.htm

[Paul Bowen]

The brand of the lens is Soligor, but it is my understanding they did not make anything so I am not sure who actually made the lens. The lens is an older lens, but appeared to have never been used with original packaging and instructions. I have seen and used a number of reflex lens, all were 500mm and with fixed aperture of f8. This is the only one I have ever seen that did not have a place for a drop in filter (if that is relevant), but did have an aperture ring marked f8, f11 & f16. As best as I can tell, rotating the ring moves something significant (large) inside the lens and I am guessing it is rotating a neutral density filter into place, but that is a guess on my part. The weird thing is that took several shots of a fixed object (a boat on water) with the camera mounted on a tripod. I used aperture mode and set the "aperture ring" on the lens to the 3 different settings. The result was that shutter speed increased as expected when going from f8 to the f11 setting or to the f16 setting, so the exposure brightness was the same for all shots. The surprising thing was that the depth of field definitely increased, which makes no sense to me if the actual iris is fixed at f8. Hence the reason for my original question.

[ajohnw]

The brand of the lens is Soligor, but it is my understanding they did not make anything so I am not sure who actually made the lens. The lens is an older lens, but appeared to have never been used with original packaging and instructions. I have seen and used a number of reflex lens, all were 500mm and with fixed aperture of f8. This is the only one I have ever seen that did not have a place for a drop in filter (if that is relevant), but did have an aperture ring marked f8, f11 & f16. As best as I can tell, rotating the ring moves something significant (large) inside the lens and I am guessing it is rotating a neutral density filter into place, but that is a guess on my part. The weird thing is that took several shots of a fixed object (a boat on water) with the camera mounted on a tripod. I used aperture mode and set the "aperture ring" on the lens to the 3 different settings. The result was that shutter speed increased as expected when going from f8 to the f11 setting or to the f16 setting, so the exposure brightness was the same for all shots. The surprising thing was that the depth of field definitely increased, which makes no sense to me if the actual iris is fixed at f8. Hence the reason for my original question.

That sort of thing as I mentioned earlier is not impossible. I rather silly way would be to have an iris immediately behind the glass at the front. I believe most of them use spherical mirrors and a correction lens or lenses at the back. In principle with the right optics arrangement at the back the F ratio could be changed. As the central mirror shades things it's also possible to have optics part way along the lens

This might not work or reveal anything but if you hold the lens up to the light and look through the rear end from different distances you may see a circle of light that changes size.

I would be curious to see a few photo's of it posted if you can. A quick search found mention of a name the poster thought was Ohmar or something like that. It seems Soligor was a German company but any optics I have seen were made in Japan. When I used the brand I would regard them as so so. I used to have a 200mm prime. I believe later on they brushed up their act a bit. Not sure. The best guide to choosing a mirror lens aught to be it's diameter. The bigger the better as it implies that the 2ndry mirror has more magnification and as a result will be "smaller". In quotes as it's % of the main mirror diameter at the back is the important aspect.. Telescope manufacturers often specify it as a % area but that makes the size look much better.

John
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