Variable aperture lenses, f-stop, and sharpness

ksparks519 · 11-05-2022, 10:48 AM

Hi all,

I have a question about stopping down to improve sharpness on a variable aperture lens that I'm hoping will help me better understand how to get the most out of an old manual zoom lens. If I zoom all the way out the aperture increases one full stop. If I zoom all the way in and set the aperture to f/8.0, is that really f/8 or is that f/5.6 will less light effectively making it through the lens? If I'm stopping down to improve sharpness and want to be at f/8, should I be setting the aperture to f/8 or f/11?

Thanks!

**Henrico** · 11-05-2022, 12:21 PM

F8 will be F8. Only wide open the aperture shifts while zooming. With a TTL metering the exposure will be correct.

**UncleVanya** · 11-05-2022, 01:03 PM

Originally posted by ksparks519

Hi all,

I have a question about stopping down to improve sharpness on a variable aperture lens that I'm hoping will help me better understand how to get the most out of an old manual zoom lens. If I zoom all the way out the aperture increases one full stop. If I zoom all the way in and set the aperture to f/8.0, is that really f/8 or is that f/5.6 will less light effectively making it through the lens? If I'm stopping down to improve sharpness and want to be at f/8, should I be setting the aperture to f/8 or f/11?

Thanks!

It depends on how you set your aperture. If you set it using the aperture ring, (on a Pentax camera) then f8 is only f8 at the shorter end of the zoom. On these lenses there's a mark showing where to read the aperture at the long end of the zoom. If you set it using the camera it will be f8 unless the lens can't maintain f8 at that position (for example, a using a 2x teleconverter on a lens that's f5.6 on the long end is now f11 on that end. Setting exposure to f8 and then zooming will charge the displayed valueo f11.

---------- Post added 11-05-22 at 04:07 PM ----------

Originally posted by Henrico

F8 will be F8. Only wide open the aperture shifts while zooming. With a TTL metering the exposure will be correct.

All aperture openings will shift value in a variable design; but as I indicated, modern cameras set to a camera controlled aperture will try to keep that setting by opening or closing as you zoom - attempting to maintain a specific value unless forced to change.

swanlefitte · 11-05-2022, 03:18 PM

The aperture will only open so much. And as the lens gets longer the ratio is shorter. A 50mm lens with an aperture or 25mm is 25/50 or f/2. If it zooms to 100 but the aperture can't open past 25mm you now have 25/100 or f/4 and that is really f/4 not f/2 with less light.
Of course it is more complicated. As entrance pupil and aperture are different do to the design and a lens is multiple lenses stacked and moved.

ksparks519 · 11-06-2022, 11:38 AM

Originally posted by swanlefitte

The aperture will only open so much. And as the lens gets longer the ratio is shorter. A 50mm lens with an aperture or 25mm is 25/50 or f/2. If it zooms to 100 but the aperture can't open past 25mm you now have 25/100 or f/4 and that is really f/4 not f/2 with less light.
Of course it is more complicated. As entrance pupil and aperture are different do to the design and a lens is multiple lenses stacked and moved.

Thanks everyone for the insights! I think the thing that was worrying me was my old manual 70-210 zoom lens. Zooming in, the aperture goes down a stop but the aperture blades themselves don't move. It made me wonder if there was a practical difference between changing aperture with the rings vs. zooming.

pschlute · 11-06-2022, 06:13 PM

Originally posted by ksparks519

I think the thing that was worrying me was my old manual 70-210 zoom lens. Zooming in, the aperture goes down a stop but the aperture blades themselves don't move

As pointed out above an f-stop is the ratio of focal length to aperture diameter (to be more accurate ...to "apparent" aperture diameter)

So on your variable aperture zoom, as you zoom from 70mm to 210mm, the widest available aperture will shift from being (as an example) f 4 to f 5.6. This can happen without the aperture physical size changing. Equally the slowest aperture available changes from say f22 to f32

Manual Variable aperture zooms often have a different index on the aperture ring which you should use when fully zoomed. Look at this picture. At 70mm you read the aperture using the long bold index line which reads f 11. But zoom the lens to 210mm and you need to use the smaller index line to the left, which is indicating f 16

11-05-2022, 10:48 AM	#1
ksparks519 Junior Member Join Date: Jun 2022 Posts: 46	Variable aperture lenses, f-stop, and sharpness Hi all, I have a question about stopping down to improve sharpness on a variable aperture lens that I'm hoping will help me better understand how to get the most out of an old manual zoom lens. If I zoom all the way out the aperture increases one full stop. If I zoom all the way in and set the aperture to f/8.0, is that really f/8 or is that f/5.6 will less light effectively making it through the lens? If I'm stopping down to improve sharpness and want to be at f/8, should I be setting the aperture to f/8 or f/11? Thanks!

11-05-2022, 01:03 PM	#3
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by ksparks519 Hi all, I have a question about stopping down to improve sharpness on a variable aperture lens that I'm hoping will help me better understand how to get the most out of an old manual zoom lens. If I zoom all the way out the aperture increases one full stop. If I zoom all the way in and set the aperture to f/8.0, is that really f/8 or is that f/5.6 will less light effectively making it through the lens? If I'm stopping down to improve sharpness and want to be at f/8, should I be setting the aperture to f/8 or f/11? Thanks! It depends on how you set your aperture. If you set it using the aperture ring, (on a Pentax camera) then f8 is only f8 at the shorter end of the zoom. On these lenses there's a mark showing where to read the aperture at the long end of the zoom. If you set it using the camera it will be f8 unless the lens can't maintain f8 at that position (for example, a using a 2x teleconverter on a lens that's f5.6 on the long end is now f11 on that end. Setting exposure to f8 and then zooming will charge the displayed valueo f11. ---------- Post added 11-05-22 at 04:07 PM ---------- Originally posted by Henrico F8 will be F8. Only wide open the aperture shifts while zooming. With a TTL metering the exposure will be correct. All aperture openings will shift value in a variable design; but as I indicated, modern cameras set to a camera controlled aperture will try to keep that setting by opening or closing as you zoom - attempting to maintain a specific value unless forced to change. Last edited by UncleVanya; 11-05-2022 at 01:10 PM.

11-06-2022, 06:13 PM	#6
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,198	Originally posted by ksparks519 I think the thing that was worrying me was my old manual 70-210 zoom lens. Zooming in, the aperture goes down a stop but the aperture blades themselves don't move As pointed out above an f-stop is the ratio of focal length to aperture diameter (to be more accurate ...to "apparent" aperture diameter) So on your variable aperture zoom, as you zoom from 70mm to 210mm, the widest available aperture will shift from being (as an example) f 4 to f 5.6. This can happen without the aperture physical size changing. Equally the slowest aperture available changes from say f22 to f32 Manual Variable aperture zooms often have a different index on the aperture ring which you should use when fully zoomed. Look at this picture. At 70mm you read the aperture using the long bold index line which reads f 11. But zoom the lens to 210mm and you need to use the smaller index line to the left, which is indicating f 16 Last edited by pschlute; 11-06-2022 at 06:45 PM.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Do internal focus lenses have variable sharpness at different focus points	dkpentax	Pentax SLR Lens Discussion	17	06-11-2022 12:00 PM
Variable aperture vs constant aperture zoom lenses	biz-engineer	Pentax SLR Lens Discussion	22	03-11-2019 07:30 AM
Aperture control problems with variable aperture lenses (with aperture rings)	gtxSeries	Troubleshooting and Beginner Help	3	09-13-2017 01:11 PM
Sharpness, Fine Sharpness, Extra Sharpness	Taviali	Pentax K-5 & K-5 II	8	03-31-2012 12:56 PM
k-7 sharpness / fine sharpness / fine sharpness 2	mattdm	Pentax DSLR Discussion	10	12-12-2010 08:10 AM

11-05-2022, 12:21 PM	#2
Henrico Loyal Site Supporter Join Date: Jun 2013 Location: Utrecht Photos: Gallery Posts: 255	F8 will be F8. Only wide open the aperture shifts while zooming. With a TTL metering the exposure will be correct.

11-05-2022, 03:18 PM	#4
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	The aperture will only open so much. And as the lens gets longer the ratio is shorter. A 50mm lens with an aperture or 25mm is 25/50 or f/2. If it zooms to 100 but the aperture can't open past 25mm you now have 25/100 or f/4 and that is really f/4 not f/2 with less light. Of course it is more complicated. As entrance pupil and aperture are different do to the design and a lens is multiple lenses stacked and moved.

11-06-2022, 11:38 AM	#5
ksparks519 Junior Member Join Date: Jun 2022 Posts: 46 Original Poster	Originally posted by swanlefitte The aperture will only open so much. And as the lens gets longer the ratio is shorter. A 50mm lens with an aperture or 25mm is 25/50 or f/2. If it zooms to 100 but the aperture can't open past 25mm you now have 25/100 or f/4 and that is really f/4 not f/2 with less light. Of course it is more complicated. As entrance pupil and aperture are different do to the design and a lens is multiple lenses stacked and moved. Thanks everyone for the insights! I think the thing that was worrying me was my old manual 70-210 zoom lens. Zooming in, the aperture goes down a stop but the aperture blades themselves don't move. It made me wonder if there was a practical difference between changing aperture with the rings vs. zooming.