Some thoughts on Hoods

Hi,
A recent thread on hoods started me thinking about something new.

Now I know a 50mm "std" lens has an apparent focal length of 50mm and this doesn't alter regardless of which camera it is used on. According to the Pentax literature, it has a field of view of 47 degrees. This is assuming you are using a film body. (In reality the angle of view will vary round the image as it is a rectangle.) For a hood to be effective, it should allow all the light in the field of view (to stop vignetting) but should cut out all extraneous light. A rectangular hood would therefore be best on std and wide angle. (The field of view on long lenses being much narrower, there is less of a problem) If you can't get a square hood, round with the relevent cutouts gets close but a plain round one lets in too much light top and bottom. Exactly where you don't want it! So for the 50mm lens, an absolutely ideal hood would also have a field of view of 47 degrees. In reality, you would make it just a little wider, say 50 degrees, to allow for the use of a filter.

In itself, this isn't a problem. However, if I now put that lens on an *istD, the apperent field of view because of the smaller focus area is the equivalent of a 75. This means that that it is about 32 degrees and therefore the hood should be a lot tighter. In other words, the "correct" hood is no longer the "right" hood and in this case, perhaps the hood from the 85/2M would be better if it is to be effective.

The main reason I am airing this is that my new 18-35 has just arrived complete with its hood. The main use is going to be on the MZ-S. Now, although it has the full image circle being an FA lens, Pentax said it was originally designed for the D. If so, did they design the hood for the D? If they did and it is "effective", it would be likely to cause vignetting at the wide settings on a film camera. I suspect that this is not so but has anybody used the lens on a film camera and if so is there a problem?

It may also seem like a silly question, but there is a removeable tab on the hood. Any ideas to what that is for?

Regards
Kim

Hi Kim

George put me right on this a while back.

You have to go back to school and try to remember your geometry (or was it physics). The angle of view of any lens will never change, it is a constant from the focal point of the lens out to infinity or back to the film plane. What happens on the *istD is the excess light (the image) from the image circle bleeds of around the smaller (than film) sensor into the nether regions. In effect you are only recording 66% of the available image on the *istD, it is this effect that transcribes to the 1.5x magnification. Therefore the recommended hood still applies.

Kim,

The removable tab on the hood (which is also on the SMC-DA 16-45, and SMC-FA 24-90 hoods) is so you can twiddle a polariser without taking off the hood

Having said that... I've nearly lost it several times and am considering taping it in place for a gig I'm hoping to do

Matt

Hi McBrian,
I was thinking physics or at least trig. Using Pentax figures, the field of view of the 55 Super Takumar lens is quoted as 43 Degrees, the 55 645 lens as 65 degrees and the 55 6x7 lens as 78 degrees. At the other end of the extreme, the 110 50mm lens only has a field of view of 24 degrees. So the angle of view does change but the focal length doesn't. An effective hood should be based on field of view not focal length. All that extra light spilling around the edge of the CCD could contain flare. Surely the whole purpose of the hood is to cut out unwanted light.

Hi Matt,
Thanks for that. I have been messing with a few rangefinders for the last couple of weeks. The hoods for those have a cutout or hole in the corner so as not to interfere with the finder. I couldn't for the life of me work out why you would need one in an SLR


Kim

If the laws of optics made our heads hurt before, add digital and we must be in migraine territory!

The 18-35 was designed to cover the field of view of the 35mm frame, so the hood must be designed for that. The digital sensor uses only the centre of that view, so we should be able to use a slightly narrower hood without problems. How much narrower is something else...

In reality, most SLR-style digital cameras do not allow easy fitting of hoods, and coating techniques are such that this is rarely a problem. It is, therefore, debateable that with the superb SMC coating we really need to bother too much about the hoods. In any event, they can be so shallow as to be almost useless.

I do use the hoods, but more to keep fingers off the front element than anything else. It's not easy to induce flare in a SMC lens. Using a hood is, IMHO, more of a "belt and braces" thing, and I tend to stand in shadows and so on to protect the lens against flare, as well as using the hood.

Kim is quite right that a tighter hood would work with the 18-35 on the D/DS. However, I think JR is also right in saying that it probably doesn't make much difference in practice. I personally have shot pictures straight into the evening sun with this lens, and seen no ill-effects whatever.

Nonetheless, I have a variety of old hoods and experiment with them on different lenses on the D. Many lenses with a larger diameter than 49mm can safely be used with step-down rings so that 49mm filters and hoods can be used without the vignetting that would occur on a film camera. For instance, my FA 2.8 100mm macro, which has a 58mm filter ring, is quite happy with a 49mm hood designed for the narrower AOV of the 3.5 135mm.

G

Hi Kim,

I reread your post and now understand what your trying to say. If I'm right this time your actually proposing to reduce the the image (light) that reach's the sensor (film plane) buy using a tighter hood, which in effect narrows the field of view to match the senor dimensions, instead of letting the excess blead away around the sensor inside the camera body.

Another thought along the same lines, somebody posted earlier about the lack of affordable long lenses for the D. Cheap consumer lenses on a 35mm body can have problems with vignetting/contrast and to a degree flare. Using these lenses on the D (or DS) might nor present so many problems especially with vignetting for the above reasons, contrast can be adjusted on the PC and flare could be controlled by being carefull where you shoot from.