rayazwinski@watmath.UUCP (Rick Yazwinski) (03/10/86)
I have read several different opinions on this subject and I was wondering which was the truth. I have heard that when using a flash and, say a 135mm lens you have to change you f-stop to allow for the light loss through the extra glass. I have also heard that f4.5 is f4.5 no matter which lens you use. PLEASE HELP!!!!! Rick...
jorge@genat.UUCP (Jorge Olenewa) (03/11/86)
In article <1587@watmath.UUCP> rayazwinski@watmath.UUCP (Rick Yazwinski) writes: >I have read several different opinions on this subject and I was wondering >you f-stop to allow for the light loss through the extra glass. >I have also heard that f4.5 is f4.5 no matter which lens you use. >PLEASE HELP!!!!! > > Rick... In order to set the record perfectly straight, the answer to this question might entail a rather long dissertation. For short, the latter statement is correct in all cases (the assumption is that you are following the indications of your flash calculator and that you DO NOT HAVE A FILTER ON THE LENS). In order to understand this better, consult the Photo books in your local library and/or photo retailer under "Flash". Cheers, Jorge L. Olenewa Genamation Inc. 351 Steelcase Rd. West Markham, Ontario L3R 3W1 (416) 475-9434
dave@onfcanim.UUCP (Dave Martindale) (03/11/86)
In article <1587@watmath.UUCP> rayazwinski@watmath.UUCP (Rick Yazwinski) writes: >I have heard that when using a flash and, say a 135mm lens you have to change >you f-stop to allow for the light loss through the extra glass. >I have also heard that f4.5 is f4.5 no matter which lens you use. It is, strictly speaking, true that f/4.5 is always f/4.5, since the f/number really is just the ratio of the focal length of the lens to its effective aperture. But the f/number does not tell you how much light the lens passes - it tells you how much light the lens would transmit if there was no loss of light due to reflection or absorption or scattering in the lens. In general, the more complex the lens, the more light lost, and the more exposure needed. But a 135mm fixed lens may lose no more light than a 50mm fixed lens, because it may have no more elements. Zooms and extreme wide-angle lenses are most likely to have loss-of-light problems. For most purposes, I doubt if the extra light loss will matter (particularly with flash, where it is difficult to set exposure exactly anyway). Some lenses are marked with "T" stops rather than "F" stops. In this case, the markings really are accurate, because the transmission of the lens has been measured and the light lost is taken into account. I only know of such lenses for specialized applications though (cinematography, view cameras). One thing you *do* have to watch out for are zooms that change aperture as they zoom - the real aperture may be one full stop below the marking on the ring. Through-the-lens flash metering avoids all of these problems.
hofbauer@utcsri.UUCP (John Hofbauer) (03/12/86)
> I have heard that when using a flash and, say a 135mm lens you have to change > you f-stop to allow for the light loss through the extra glass. > I have also heard that f4.5 is f4.5 no matter which lens you use. > Not true, in principle. In practice, though, you may find it necessary to open up the lens further, but not because of the extra glass in the lens. Indeed, really long lenses (>400mm) may only have two or three elements. When using a flash in manual mode and calculating the exposure from first principles you may compute an incorrect f-stop because of violated assumptions. Flashes are rated by a Guide Number -- the higher the number, the more powerful the flash. This is not a very scientific measure but good enough in practice. The f-stop can be determined by dividing the Guide Number by the distance. The Guide Number, though, assumes a normal size room with average reflectance. Therefore if you shoot outdoors at night you should open up a stop or two to compensate for the lack of reflectance since the flash essentially becomes a point source of light. Another problem, which is more likely to arise with the use of telephoto lenses, is using the flash beyond its normal range. A flash can only deliver so much illumination and this, of course, is governed by the inverse square law. It never fails to amuse me to go to a grand outdoor event, such as a rock concert, and watch people popping off their flash cubes. It makes for a lovely light show but doesn't do the would-be snapshooters a bit of good. With a long lens you are likely photographing a subject which is farther away and so it may be necessary to open up the lens more to obtain a correct exposure. Underexposure is generally due to a combination of these two factors. P.S. I hope no one brings up the subject of T-stops.
tsp@hadron.UUCP (T. Scott Pyne) (03/13/86)
[] In article <1587@watmath.UUCP> rayazwinski@watmath.UUCP writes: >I have heard that when using a flash and, say a 135mm lens you have to change >you f-stop to allow for the light loss through the extra glass. >I have also heard that f4.5 is f4.5 no matter which lens you use. The latter is true. f4.5 is actually f/4.5, meaning that the aperture = the focal length / 4.5. Thus, the actual aperture is larger with a 135mm lens than with a 50mm lens for a given f/ stop. If the calculated f/ stop for a given flash-subject distance is f/4.5 then it will be f/4.5 for any focal length lens, modulo certain situations involving extension tubes or bellows. Scott Pyne ...!seismo!hadron!tsp
dberg@noscvax.UUCP (David I. Berg) (03/13/86)
In article <1587@watmath.UUCP>, rayazwinski@watmath.UUCP (Rick Yazwinski) writes: > I have heard that when using a flash and, say a 135mm lens you have to change > you f-stop to allow for the light loss through the extra glass. > I have also heard that f4.5 is f4.5 no matter which lens you use. > PLEASE HELP!!!!! To answer the second question first, yes f4.5 is f.45 is f4.5. The f stop of a lens (any lens) is the ratio of its focal length to the diameter of the lens aperture. It measures the amount of light which will be admitted to the flim plane. Since the intensity of light decreases with the square of the distance it travels, the longer the lens, the larger the diameter of the aperture must be to achieve the same amount of light intensity at the film plane. That is why a 400mm f4.5 lens has a larger diameter front element than a 200mm f.45 lens. (You will notice that each f stop is the square root of two larger than the next smaller f stop. Eg. f1.4 is 1*sqrt2; f2 is 1.4*sqrt2; 2.8 is 2*sqrt2, etc. This is a function of that exact principle.) With regard to the flash question, there is light loss in a telephoto lens (partially due to the above stated principle, and partially due to the abosrption by the additional lens elements). I'm not sure that a 135 mm lens would cause enough light loss to cost an f stop. You might want to experiment with this. -- David I. Berg (dberg) ARINC Research Corporation San Diego, CA MILNET: dberg@nosc UUCP: {ihnp4 akgua decvax dcdwest ucbvax}!sdcsvax!noscvax!dberg
darrelj@sdcrdcf.UUCP (Darrel VanBuer) (03/15/86)
>>I have also heard that f4.5 is f4.5 no matter which lens you use.
In an ideal world, this might be true, but in actual practice, there is
enormous variation. 15-20 years ago, Nikor lenses were very careful to have
the marked F-stops very close across the lens line because through the lens
metering was not yet common. Needed if you wanted your handheld (or
Photomic-F) meter readings to come out consistent across a collection of
lenses. Thus my ancient Nikor 500/5 mirror lens is 122 mm in diamater
(enough to call 4.2 in todays market!) to compensate for the large hole in
the middle of the lens.
The worst lens I have in this regard is an off-brand cheapo 85-210/3.8 zoom
which meters a full stop slower than my Nikor 200/4 (i.e. it's a full 1.3
stops slower than "marked"). A combination of lots of reflection loss from
the high number of elements, probably limited use of coating (claims
"Tri-Coat" but maybe thats in the whole lens?), and marketing bias in
roundoff (engineer says it's formulated at 3.85000, sales says round "down").
This is also noticable in many mirror lenses now (500/8s taking 72mm
filters, indeed!
[I originally thought the problem with the cheapo zoom was poor sharpness
because the viewfinder image looked so awful, till I noticed it was metering
a stop slower; the 200/4 Nikor looked the same stopped down to 5.6]
--
Darrel J. Van Buer, PhD
System Development Corp.
2525 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
!sdcrdcf!darrelj
VANBUER@USC-ECL.ARPAsmh@mhuxl.UUCP (henning) (03/16/86)
**** **** From the keys of Steve Henning, AT&T Bell Labs, Reading, PA mhuxl!smh In article <1587@watmath.UUCP>, rayazwinski@watmath.UUCP (Yazwinski) writes: > I have heard that when using a flash and, say a 135mm lens you have to > change your f-stop to allow for the light loss through the extra glass. > I have also heard that f4.5 is f4.5 no matter which lens you use. It is not amount of glass, but the number of interfaces that cause light loss and flare in most lenses. It turns out that the Minolta 135mm lens has the least number of elements, 4, of any Minolta lens thus the least loss. The extreme wide angles have a large number of elements, 11 or 12 and the zoom lenses have the outlandish number of elements 14-16. I do 100mm flash photography all the time and never have a problem.
seifert@hammer.UUCP (Snoopy) (03/19/86)
In article <1587@watmath.UUCP>, rayazwinski@watmath.UUCP (Yazwinski) writes: > I have heard that when using a flash and, say a 135mm lens you have to > change your f-stop to allow for the light loss through the extra glass. > I have also heard that f4.5 is f4.5 no matter which lens you use. [ various followups about T-stops and the number of elements in various lenses ] Isn't anyone going to point out the obvious? You need a tele-lens on the flash to concentrate the light on the area covered by the lens. Some flashes have various focal length lenses available for this purpose. The more important case is when you're using a wide angle, otherwise the center will be overexposed and the edges underexposed. {not to ignore 'T-stops' and the number of elements in a lens, as this could be significant in some cases. If it is significant, it will also be significant if you use an external (non-TTL) light meter. I would think that TTL flash *should* compensate for this.} Snoopy (an "available darkness" photographer) tektronix!tekecs!doghouse.TEK!snoopy (That's TTL = Through-the-lens metering, not transistor-transistor-logic)
smh@mhuxl.UUCP (henning) (03/24/86)
**** **** From the keys of Steve Henning, AT&T Bell Labs, Reading, PA mhuxl!smh > In article <1587@watmath.UUCP>, rayazwinski@watmath.UUCP (Yazwinski) writes: > > I have heard that when using a flash and, say a 135mm lens you have to > > change your f-stop to allow for the light loss through the extra glass. > > I have also heard that f4.5 is f4.5 no matter which lens you use. > > Isn't anyone going to point out the obvious? You need a tele-lens on the > flash to concentrate the light on the area covered by the lens. "need"?????? You don't NEED a tele filter for the flash unless you don't have enough light otherwise. You can use the tele filter, but you normally don't need it. Interestingly they suffer more from light loss than you 135mm tele lens does. The 135mm telefilter for a flash should give you 2.5 to 3 stops more light but in fact usually gives 1 stop.