With what Telephoto Lens might One Achieve such a Photo...?

[rick55]

Gidday again,


This is a conversation I have with students, every year . . . truely . . . maybe the best way to satisfy yourself is to shoot the JPEGS for yourself, that's what I say the them.

Cheers,

WW

PS: they really are JPEGS straight out of the box the EXIF is attached for you to interrogate – and you will see, if you look at the date and time I did just get home from my very late Saturday night shoot – and it is early Sunday morning ... and I am quite mad.

Wow, thanks, Bill! Not mad, more like fascinated. I am, too.

I certainly believe your numbers, but won't the camera process the light levels when it creates the jpg? If I look at the image from across the road, it certainly appears that there's a lot of shadow detail missing, but that may be the jpg process.

I'll certainly try to do some experimenting, within the limits of the two lenses I have for my 500D: I can't compare between the two, because they have different entry pupils.

I wouldn't be surprised if the illumination drops off more slowly than inverse square would predict: the nature of some reflected light probably causes it to act somewhat like a beam. But I'll be plenty surprised if it's constant over distance. Of course, I've been surprised lots of times before.

Cheers!
Rick

[William W]

but won't the camera process the light levels when it creates the jpg? If I look at the image from across the road, it certainly appears that there's a lot of shadow detail missing, but that may be the jpg process.

Yeah well remember that is x9 enlargement - I didn't have a 1200mm lens . . .

If I shot at 17 yards with a 400mm lens I'd have about the same FoV as the 50mm at 2 yards . . . and a shot at 17 yards with a 400mm I would expect the same shadow detail as I got with the 50mm lens.

***

But I'll be plenty surprised if it's constant over distance. Of course, I've been surprised lots of times before.

OK, Yes there IS falloff.

I was kind of avoiding (getting into) the detailed physics and longer distances . . . but for practical purposes and shooting distances with different lenses - there is no appreciable difference to exposure – (even shooting up to 2KM – as in the crop of the window from my first set of the Moon setting)

As noted if we CROP, as with my 9x enlargement, there will be a problem with detail - yes the shadows go first, cause we are "spraying it thinner" (ref your pixel argument) when we make an effectively tighter FoV by cropping - BUT even in the enlargement THE GREY CARD / EXPOSURE was still the same as in the 50mm shot.

I gotta go to bed,

CU round,

WW

[rick55]

Sorry, I thought I posted this, but I must have hit "back" or something.

I think I see where I was confusing myself: I forgot that f-stop is focal length over physical aperture. Duh! When I started thinking about the two lenses I use, I remembered that for a fixed f-stop zoom, as you zoom in to make the same frame, the lens is opening the diaphragm to keep the same f-stop. So it's letting in more light.

Maybe the fall-off you mention is the fact that f-stop is physical aperture diameter, whereas light-gathering is the area of the aperture. Anyway, it makes me happier to have the real-world behavior at least approximate theory. I always get nervous when there's a substantial disagreement: much more likely it's my lack of understanding.

Cheers!
Rick

[William W]

I forgot that f-stop is focal length over physical aperture. Duh! When I started thinking about the two lenses I use, I remembered that for a "fixed f-stop zoom", ** [i.e. "a non varying maximum aperture zoom"], as you zoom in to make the same frame, the lens is opening the diaphragm to keep the same f-stop. So it's letting in more light.

**[My inclusion / clarification to confirm what we are discussing]

Now we are scoring goals.

I was "avoiding" the physics because I didn't know how "deep" you wanted to go. Many folk are not comfortable with the mathematics - I wanted to make a practical demonstration, to show you.

Maybe the fall-off you mention is the fact that f-stop is physical aperture diameter, whereas light-gathering is the area of the aperture.

Kind of part of it, yes. We need to introduce the T-Stop

As a general rule for zoom lenses: As the lens’s Focal Length increase, the T-Stop for particular "aperture" of any zoom lens usually has greater variance to that particular lens's F-Stop at that particular aperture.

Similarly, for Prime Lenses there is usually greater variance between T-Stop and F-Stop, as the Prime lenses get into telephoto Focal Lengths, compared to Standard and Wide angle Primes.

Noted also that the longer Focal Lengths are generally used for shooting Subjects which are at a distance from the camera

That's part one main part of it of it. Another main part is simple atmospherics the air is a big (mini) ND filter - the further away the more ND - if that makes sense.

These are minute differences - but added together can be 1/3 stop maybe 1/2 stop when comparing using a 50mm lens and the previously mentioned 1200mm lens each shooting a full frame portrait (with the same FoV – i.e. one close to me and the other at 50 yards) . It can be more apparent with big mirror telephoto lenses or when using small but good quality telescopes to "act as" a telephoto lens.

Also Cinematographers are more critical about T-stops: for Still Photography, now mainly digital and considering the range of lenses we generally use and the Compass of distances our subjects usually are from us – the actual exposure difference we would never notice or ever take into account.

Back to a practical example – I mentioned earlier that there were (at least) three EF1200L at the 2000 Olympics in Sydney and these lenses were at a minimum used for the rowing . . . it would not surprise me if the exposure was increase maybe 1/3 stop at morning or at dusk, or maybe ½ a stop if using the x1.4MkII tele-converter at those times simply because of the atmospherics

What I mean is the guy using the 1200L+ x1.4MkII could be pulling F/11 @ 1/1000s @ ISO1600 for perfect skin one exposure and the his partner on the shore using a 85mm lens could be pulling:
F/4.5 @ 1/1000 @ ISO800 and have exactly the “same” exposure on skin tones.

But usually on a football field if we were working in tandem and I am at one end, running, with a 70 to 200mm and a 300mm and you are at the other end in the corner with the 500mm on a pod and we talk on the walkie-talkie ,you might ask me to check you gear “what light do you have?” – it will always we have “the same light” – and we will be pulling the same exposures through the match – the fact that you are usually 50 to 75 yards further from the action than I is irrelevant.



***

We have discussed cropping a small portion of the image and enlarging that to get an equivalent FoV - that's different again – because we are introducing other factors.

WW

[carregwen]

I saw a nice seagull this afternoon...

[rick55]

I saw a nice seagull this afternoon...

How far away was it, and what did your spot meter read?

[pwnage101]

I forgot that f-stop is focal length over physical aperture.

yep, you hit it on the nail. that's why you see an enlargement of the entrance pupil as you rotate the focal length ring on a zoom lens. these enlargements are more pronounced on lenses with a fixed minimum aperture (none of this f/4.0-5.6 nonsense!)

[comicsdecoder]

Robert

This sounds less Ballard and more Hitchcock. Have you seen 'Rear Window'? http://uk.imdb.com/title/tt0047396/ And I still want to know who posted this shot using my username. Let's get undressed and watch the boys...

But... if you are serious about this, then forget the EF1200 (that's for amateurs) and get yourself this little(?) beauty for $55,000! http://soldierllk.multiply.com/journ...200mm_f14_lens

EDIT: Sorry, mate, forgot to add the

Oh, yes! Rear Window is a classic. Like Blow Up, it's one of those films that's sure to be a hit with you photography buffs!

I'm really impressed with the responses my (sort of) hypothetical question has gotten. If I were a professor of photography, I'd feel really satisfied with the participation of my pupils! This looks like it's been a really good exercise all around.

A lot of that equipment to which you fellas have been providing links is truly impressive as well--you enthusiasts must get extremely jealous and covetous! Some of those lenses look powerful enough to not just enlarge a helicopter, but actually bring the damn thing down!

Cheers!