Fast lens question ???

Hi
At the risk of sounding stupid can anyone tell me the benefits of paying all that money on a fast aperture lens.
If for instance the sweet spot for my 100-400mm is F7.1, F8.0 ish why would I pay to get a F2.8 lens.
How come if when I take a macro pic with a F2.8 the depth of field is so shallow ? Is the depth of field that shallow on say, a 400mm F2.8 ? I have taken shots of a slug where its eyes are sharp but its back end is completely out of focus. To combat that I step down to F8.0 which seems to negate the benefit of a large aperture.
How does the depth of field work on these lenses, it obviously does but how
Thanks
Ian

h14nha wrote in post #10267175
At the risk of sounding stupid can anyone tell me the benefits of paying all that money on a fast aperture lens.
If for instance the sweet spot for my 100-400mm is F7.1, F8.0 ish why would I pay to get a F2.8 lens.
How come if when I take a macro pic with a F2.8 the depth of field is so shallow ? Is the depth of field that shallow on say, a 400mm F2.8 ? I have taken shots of a slug where its eyes are sharp but its back end is completely out of focus. To combat that I step down to F8.0 which seems to negate the benefit of a large aperture.
How does the depth of field work on these lenses, it obviously does but how
Thanks
Ian

Since you said it first, yes, you don't sound like you know a lot about photography.

Since DOF depends on subject distance, you will have a lot less DOF when focusing in the macro range vs. focusing on something far away. Play with various numbers in this free DOF calculator and you'll see for yourself:
http://www.dofmaster.c​om/dofjs.html

Fast apertures have many benefits, including (but not limited to):
1. Allowing faster shutter speed to avoid camera shake
2. Allowing faster shutter speed to avoid subject movement
3. Allowing faster/more precise AF (most of Canon's newer DSLRs can take advantage of lenses F2.8 or faster)
4. Allowing brighter viewfinder to allow easier MF or finetuning after AF
5. Shallower DOF when you need it (you can always stop down for more DOF, but cannot open up beyond the lens' maximum aperture)
6. More reach with flash (doubling with every 2 stops of aperture)

Finally, high-end lenses are built to be sharp wide-open or close to wide-open, whereas cheaper lenses need to be stopped down 2-3 stops to be optimal. For example, at the wide end, the 17-55/2.8 seems to have less than one stop of advantage over the 18-55/3.5-5.6, but to get the same sharpness, the 18-55 will likely have to be stopped down to F5.6 or even F8 to match the 17-55 at F2.8. So really, you are talking about 2-3 stops of difference.

In the end, if you can't see the benefit of spending that extra money on fast lenses, don't. It would be a waste (both ways)....

h14nha wrote in post #10267175
Hi
At the risk of sounding stupid can anyone tell me the benefits of paying all that money on a fast aperture lens.
If for instance the sweet spot for my 100-400mm is F7.1, F8.0 ish why would I pay to get a F2.8 lens.

Well, I can't really see a difference between wide open (f/5.6) and f/8.0 on my 100-400. And my 135L is just as sharp at f/2 as my 100-400 is at f/8.

Just because some lenses get a hair sharper as you stop down doesn't mean they should not be used wide open and it has nothing to do with the usefulness of a fast lens.

Most macro lenses are slow for exactly the reason you noted, at macro focus distances it is hard to get enough DOF and most macro shots will be stopped down.

This does not necessarily apply to other fields of photography. I shoot sports quite a bit, and my lenses are almost always wide open when I do.

[QUOTE=RetroBlader;102​67250]Since you said it first, yes, you don't sound like you know a lot about photography.

^^^
Didn't need to reiterate

Since DOF depends on subject distance, you will have a lot less DOF when focusing in the macro range vs. focusing on something far away. Play with various numbers in this free DOF calculator and you'll see for yourself:
http://www.dofmaster.c​om/dofjs.html

^^^
Useful thanks

In the end, if you can't see the benefit of spending that extra money on fast lenses, don't. It would be a waste (both ways)....

^^^
Totally uncalled for


I do understand large apertures v fast shutter speeds, my question was aimed more at depth of field of large aperture lenses. I asked a polite question and I didn't expect to be insulted in any replies.

Hi Jeffery,
Thanks for your reply, I just had a look at your Flickr page. You have some great pics on there, that Maple leaf is absolutley stunning, my favourite of them all.
I do open up my 100-400mm when the light requires it, but try to keep it stopped down a bit and use the ISO to get a fast shutter speed as I like to take pics of birds amongst other things.
Cheers Ian

h14nha wrote in post #10271021
[my question was aimed more at depth of field of large aperture lenses.

Your question seemed to be based around 'why bother if you have to stop down to get enough DoF'. The big advantage of fast lenses is when you specifically want to go the other way and work with a shallower DoF, to separate the subject from foreground / background. The 100-400L can put the background out of focus to a degree at f.5.6 but the 400 f/2.8 (wide open) will be able to make the background much more OOF and reduce blurry detail to much more of a smooth toned background with few, or no, obvious shapes. That can be the difference between a beautiful background at f/2.8 and a messy one at f/5.6

You can always stop a fast lens down to get more DoF, but you can't open up a slow one to get really shallow DoF.

Also, as pointed out above, most (if not all) canon DSLRs have at least one high precision focus point that only works at f/2.8 or faster. That can make or break a shot in low light where the AF is struggling to see what it is doing with a slow lens.

Add in, as you are aware, the ability to use faster shutter speeds to avoid unwanted blur and camera shake and the reasons should be clear as to why fast lenses are so popular.

I too have the 100-400L, but find that I often run out of light when shooting with it in the field and struggle to get enough shutter speed. I bought the 300 f/2.8L to get around that and the difference it makes is amazing.

My kit is based around 3 zooms for versatility and a set of fast primes for when I really need faster shutter speeds than i can get with the zooms, as well as more accurate AF.

All decent lenses are sharp enough wide open. Yes, they will become a touch sharper stopped down a little but that is only really noticeable when pixel peeping. After processing and sharpening that pretty much goes away anyway, and once outputted to a normal size print or web image you will never tell them apart.

h14nha wrote in post #10271021
I do understand large apertures v fast shutter speeds, my question was aimed more at depth of field of large aperture lenses. I asked a polite question and I didn't expect to be insulted in any replies.

Sorry, I did not mean to be insulting. (I wanted to have some "zing", but definitely not to the point of insulting -- otherwise I would have used the same "s" word you used.)


1. Sweet Spot/Image Quality
As mentioned, the "sweet spot" of a high-end lens is different from that of a lower-end lens. For example, let's look at 300mm. The 300/4 prime costs US$1269, while the 70-300IS zoom costs US$549. (I am going to leave out the very expensive 300/2.8L and the cheapo 75-300 out of this.) At 300mm, the 70-300IS is F5.6 wide-open. You might find yourself wondering why anyone would pay more than double for one extra stop of aperture, especially when it doesn't even zoom! However, let's look at the image quality data:

The 70-300IS is fairly sharp wide-open (F5.6 at 300mm): 1746 centre, 1695 corner. However, its "sweet spot" is at F8, where things improve to 1888 centre, 1741 corner.
http://www.photozone.d​e …st-report--review?start=1

Now let's take a look at the 300/4L IS. Even wide-open at F4, it's already sharper than the 70-300 at its sweet-spot: 1904 centre, 1837 corner, so the difference is more than 2 stops.
http://www.photozone.d​e …st-report--review?start=1

Not to mention, in addition to image quality gains and all of the large-aperture benefits I mentioned in my earlier reply, with the 300/4L IS you also get (way) faster focusing, full-time manual focusing, and better construction. Those benefits cost money too.


2. DOF
Sure, the DOF will be different at F4 vs F8. For example, you see a deer 100ft away. DOF will be 10.2 ft with the 70-300 at F8, and 5 ft with the 300/4L IS. In both cases, enough to have the whole deer in focus. However, the 300/4 will be able to isolate the deer more since the background will be more blurred.

Remember, most people who buy fast telephoto lenses use them on far-away objects. In those situations, shallow DOF is often desirable, as it isolates the subject from the background. Moreover, one can always get more DOF with a large-aperture lens (by stopping it down) but cannot get less DOF with a small-aperture lens.


3. Macro
Finally, about macro. Since DOF is often so shallow with macro (fractions of an inch), macro lenses are often optimized for F5.6 or F8, to allow more DOF but still providing the best image quality:
http://www.photozone.d​e …st-report--review?start=1

However, you'll find some amazing macro photos taken at F11 or even F16. At those apertures, diffraction is beginning to degrade sharpness, but the gain in DOF more than offset the loss. Here is a good article on the various trade-offs one often needs to make with macro photography:
http://www.cambridgein​colour.com/tutorials/m​acro-lenses.htm

Anyway, the Lens forum is the most visited forum on POTN. Your thread has been viewed 176 times, yet only 2 people bothered to reply. You can decide for yourself whether this means your question was so difficult that no one could answer, or that it was too elementary that no one bothered to answer.

There is always something to learn about photography. Good luck with your journey.

^^ The above post pretty much sums it up!

But to word it simpler, you pretty much pay for:
1) Increased image quality at wider apertures- most lenses are usually at their sharpest when "stopped down" (aperture set to a larger number than it's lowest- for example, f/5.6 on a f/1.8 lens). A "faster" lens will have be stopped down less to get that maximum sharpness
2) More light- wider apertures allow more light (for example, f/2.8 is just barely useable indoors while f/5.6 will be impossible to use without a flash (or getting blurry pictures))
3) Greater depth-of-field control: You have greater control on subject isolation (blur the background more to your pleasure, for example, in portraits)
4) Better for sports/moving subjects: Sports involve fast-moving subjects, which require a very high shutter speed (for example, 1/800). In order to get enough light to accompany such a high shutter speed, a very wide aperture is required to get enough light for your high shutter speed. For example, the $7000 400mm f/2.8 vs the $1600 100-400 f4.5-5.6. On the long end, you will be able to get a MUCH faster shutter speed with an aperture of f/2.8 versus f/5.6.

Is it worth it? That depends on your shooting style and how much you expect out of your pictures. If you are just coming from a point-and-shoot camera, you might be just happy with a 18-55 kit lens, while many of us must have the $1050 17-55 f/2.8 IS , going after every little pixel of sharpness


Now regarding Macro, Depth of field (how much is in focus) gets thicker at further distances, and gets incredibly thin at closer distances.
It also gets thinner with increasing focal length (mm) and gets thicker with lower mm.

Heres an example:


55mm, 2.5 inches away from the subject, f/5.6

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[QUOTE=RetroBlader;102​71408]Sorry, I did not mean to be insulting. (I wanted to have some "zing", but definitely not to the point of insulting -- otherwise I would have used the same "s" word you used.)

Forgiven,
You've explained what I wasn't sure of extremely well. I see now the actual depth of field of a couple of feet at distances is great for isolating the subject to give a great bokeh. That was what I wasn't sure of, ie, would you have to step down your aperture to get the best image quality. Perhaps I didn't explain myself very well.

Thanks to everybody who replied, I learned something from you all.

Cheers,

Ian.

Forthewinwin, those picture are superb and demonstrate what RetroBlader explained very clearly. Thanks.