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Home: Video University Forums: Digital Photography for Videographers:
advantages of a large constant aperture

jarod126
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Dec 5, 2005, 6:54 PM

Post #1 of 14 (1395 views)
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advantages of a large constant aperture	Can't Post

If anyone can help me I would appreciate it. I can't find the answer on the net.

I have found that my lens (sigma 24-70 f2.8) is a great lens, but now at times i want to achieve focus a little broader -meaning bigger depth of field. But I can't since it is a constant f2.8.

So my question is why would anyone want a constant large aperture? Wouldn't it be better to get a 2.8-5.6 or something like that to get the option to use a smaller aperture?

Alan

jt53787
Novice

Dec 5, 2005, 8:30 PM

Post #2 of 14 (1392 views)
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Re: [jarod126] advantages of a large constant aperture [In reply to]	Can't Post

The term 'constant aperture' as you are using it isn't quite correct.

Your sigma 24-70 f/2.8 should be adjustable in lower apertures on the camera. A lens that states an aperture of 2.8-5.6 means that as you zoom in to say 70mm you loose light and can only shoot as wide as 5.6 at 70mm while as you zoom out it is adjustable to 2.8 at 24mm. (using your current sized lens as an example)

What camera are you on?

did that make sense? You just need to learn how to adjust that lens that you have to a lower aperture within the camera settings.

jarod126
Novice

Dec 5, 2005, 10:48 PM

Post #3 of 14 (1380 views)
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Re: [jt53787] advantages of a large constant aperture [In reply to]	Can't Post

thanks for replying jt53787,

it is only a f2.8 lens, not a f2.8-5.6. the manual says it is a constant f2.8 lens, so I can't change the aperture.

i'm on a canon 20d by the way.

so am i missing something? is a constant aperture lens have any advantages i am unaware of?

alan

jt53787
Novice

Dec 5, 2005, 11:28 PM

Post #4 of 14 (1375 views)
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Re: [jarod126] advantages of a large constant aperture [In reply to]	Can't Post

definitely sounds like you are missing something here.
The lens is adjustable to lower apertures, it just can't open up any more than 2.8

Throw your camera into manual mode. Then use either the front scroll wheel or the large wheel on the back to adjust the aperture. (don't remember which one it is. I'm on nikon)

Storm can you jump in a help him out? You've got a 20D.

Just went onto the sigma sight and even though it says 'constant' in the description it states below in the chart that it has a minimum aperture of f22. So I'm sure it's adjustable.

Jeko
Enthusiast

Dec 6, 2005, 12:05 AM

Post #5 of 14 (1371 views)
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Re: [jarod126] advantages of a large constant aperture [In reply to]	Can't Post

f2.8 merely indicates the bottom limit of your aperture, which allows plenty of light in making this a "fast" lens.

The "constant" aperture means that it can reach that size (f2.8) throughout its zoom range. Other lenses that are not "constant" will lose that ability as the lense zooms in.

For your specific lens, the bottom aperture at 24mm is f2.8 and the bottom aperture at 70mm is also f2.8.
Another lens might be f2.8 at 24mm, but then only reach f5.6 at 70mm zoom. This would translate to a slower shutter required when zooming (to get sufficient light to the sensor).

Jeko

Sony VX2100's, iRivers, M-audio 24/96, Canon 5Ds/20Ds (and too much glass), Vegas6, PhotoShop CS3, Lightroom, etc.

jarod126
Novice

Dec 6, 2005, 4:11 PM

Post #6 of 14 (1353 views)
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Re: [Jeko] advantages of a large constant aperture [In reply to]	Can't Post

I see. Now I get it. Thanks Jeko and Jt. It makes sense now.

Alan(jarod126)

kedan
Enthusiast

Dec 13, 2005, 2:11 PM

Post #7 of 14 (1311 views)
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Re: [jarod126] advantages of a large constant aperture [In reply to]	Can't Post

yeah, you are never locked into 2.8, you can have it anywhere between 2.8 and 32 or whatever the smalles opening is.

Waldemar
Enthusiast

Dec 14, 2005, 8:20 PM

Post #8 of 14 (1295 views)
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Re: [jarod126] advantages of a large constant aperture [In reply to]	Can't Post

If you indeed have a lens that maintains its f2.8 aperture at ANY focal length you have one top quality professional lens, and i will willingly trade you any f3.5-5.6 on the market for it. i'll buy that cheap lens for you and pay shipping to you. I'll still have the best end of the bargain.

Perhaps this will help. It has been a while since i have discussed this, so I may have the numbers wrong:
The f-stop of a lens is a mathematical relationship of the aperature relative to its focal length. A 2.8 lens means the focal length is 2.8 times the aperture. Change focal length, as one would do on a zoom lens, and the relationship changes. In 35mm camera lens terms, a 28mm (focal length) wide angle lens can maintain a 2.8 aperture with the largest glass lens element being somewhere around 40mm in DIAMETER, and can accept front filters of about 52mm diameter (cheap). A short telephoto lens of 80 mm focal length with a 2.8 aperture will need its largest glass lens element to have a diameter of some 70mm diameter, requiring a filter size of 82mm diameter (really expensive).

To offset cost, lens manufacturers limit the physical maximum diameter of the lens which shifts the widest f-stop available. A 28mm-70mm 2.8-4.5 lens (3x) advertises an f-stop of 2.8. That is true for the 28mm focal length, but the maximum available f-stop is only 4.5 at 70 mm focal length. The price for such a lens would be under $100. An identical lens that maintains an f-stop of 2.8 throughtout its entire focal length will cost well over $1000.00.

Regardless of what kind of lens you have, realize YOU set the aperture. Most lenses, regardless of quality, are at their sharpest edge to edge at the mid level, usually f4.0 - 5.6. If you want more depth of field, set the aperture at f8.0, f11, f16, f22. If you want less, choose f3.5, 2.8, 2.0, whatever the widest/lowest number setting is available.
G5 1.8. Final Cut/QT Pro/iMovie/Still Life. GL1. XL1-s

Storm Crow
Veteran

Dec 15, 2005, 8:33 AM

Post #9 of 14 (1274 views)
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Re: [jt53787] advantages of a large constant aperture [In reply to]	Can't Post

In Reply To

Storm can you jump in a help him out? You've got a 20D.

Ok, this is somethign I just learned this last Saturday because I am not a still photo expert by a long shot. Yes you can adjust your aperature on your 20D. Put the camera in manual mode and then at the top of the camera right side there are 4 black buttons. press the DRIVE-ISO button and then using the large wheel on the back adjust the aperature in the display on top of the camera. Mine can go from 2.8 - 32 so yes you have a wide range to play with!
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Good Friend, around these hearth stones, speak no evil of any creature.

Warning: Exposure to the Son can prevent burn!
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videobear
Veteran

Dec 15, 2005, 8:56 AM

Post #10 of 14 (1273 views)
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Re: [jarod126] advantages of a large constant aperture [In reply to]	Can't Post

jarod, I think you're confusing two things, both of which are defined in terms of f-stop.
The "speed" of the lens is the spec you're quoting. A lens that's rated at f/2.8 lets a lot of light through its glass, while lenses with higher figures reduce the camera's light sensitivity. While the reduction in f-stop at the telephoto end of the lens will REQUIRE you to open up the camera's iris, it in itself doesn't affect the depth of field.

Then there's the camera's own exposure setting. How much light the iris will let through is also measured in f-stop.

What affects the DOF is the fact that your camera's iris has to be opened up more at the lens's telephoto end to admit enough light for proper exposure. In fact, anything that makes you open up the iris will have the same effect...for example, putting a neutral density filter on to cut the incoming light level. So if you really do have a lens with a constant f stop rating (unlikely, as other posters have said), you can still get your iris open and affect your DOF.

Regards,
Doug Graham
Panda Productions

Scott Brooks
Veteran

Dec 15, 2005, 6:55 PM

Post #11 of 14 (1253 views)
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Re: [Storm Crow] advantages of a large constant aperture [In reply to]	Can't Post

In Reply To

Storm can you jump in a help him out? You've got a 20D.

You do not have to use manual mode for this. You can set it to AV (aperture priority) and change your setting there. The camera will determine what shutter speed you need for the aperture you've selected and show you the setting in the viewfinder. If your shutter speed is too low then you need to increase your ISO setting or open up the aperture.

Storm Crow
Veteran

Dec 16, 2005, 9:14 AM

Post #12 of 14 (1227 views)
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Re: [Sparky] advantages of a large constant aperture [In reply to]	Can't Post

Very cool...thanks Sparky. Some day when I have time I need to come down there and meet you since we are both form IL.
_________________________________________________

Good Friend, around these hearth stones, speak no evil of any creature.

Warning: Exposure to the Son can prevent burn!
_________________________________________________

Scott Brooks
Veteran

Dec 16, 2005, 12:10 PM

Post #13 of 14 (1211 views)
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Re: [Sparky] advantages of a large constant aperture [In reply to]	Can't Post

When I said shutter speed too low, what I should have said was shutter speed too SLOW. In other words ... possibly too slow to be hand held without getting a blurred photo. Depending on the distance I can usually go to 1/60th, but even then it can be hit and miss. Unsure

JMCP
Novice

Dec 19, 2005, 4:01 AM

Post #14 of 14 (1157 views)
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Re: [videobear] advantages of a large constant aperture [In reply to]	Can't Post

Folks,

2.8 straight through lenses don't have to be very expensive, both SIGMA and TAMRON do lenses for less than �280 in a range similar to the original posters 24 - 70 lense.

Cheers John