[Rondec]

Lenses tend to be at their best stopped down a couple of stops. This means that if you compare an f1.4 lens shot at f2.8 to a f2.8 lens, it probably will be sharper, have less vignetting than the slower lens. If you are going to stop down a bunch to f8, then they will probably be pretty similar.

Zooms are a little different and will tend to have some distortion at certain points in their focal range that you may have to deal with. This will probably be more of a problem with zooms that have a bigger range and also cheaper, slower zooms tend to have more of an issue.

[pepperberry farm]

Something I've been pondering, mainly because I have realized that shooting group portraits you can't get away with wide open apertures any way, so the value of the fast lenses seem over-rated to me.

Frank

not everybody shoots group portraits, so the extra light comes in handy for some of us....

[aslyfox]

I know my limitations - I am no expert

I like lens with the largest ( fastest ) apertures because I believe in the idea of: if you don't try, you won't get the photo

so I tend to try to photograph in low light if necessary to get the shot

[StiffLegged]

No not Tony, Roger in the link I posted earlier.
"The main takeaway from this is that zooms are less interchangeable than primes, copy variation is much more noticeable. If you make a general statement about zooms (like I did above for the Canon 24-70s sharpness) be very aware there will be a lot of copies that behave differently.

This is how zooms are. If you ask me to find a zoom that has no field tilt or centering change throughout the range, I’d tell you it’s not worth me testing a hundred copies in the faint hope of finding one."😁

I was right, then? As someone who doesn’t “pass collimated light through insert geek apparatus here” you’re welcome!

[rogerstg]

Something I've been pondering, mainly because I have realized that shooting group portraits you can't get away with wide open apertures any way, so the value of the fast lenses seem over-rated to me.

For you, the value is probably overrated, IOW, not worth the price difference. In the days of film maxing at ~1000 ISO (with lots of grain), a fast lens allowed the use of slower film, faster shutter speeds and/or less intense flash when shooting indoors, especially when there was some activity. It was especially valuable when shooting a wedding or birthday party. Now, with high ISO sensors, the value differential may be less, but still exists. It really depends on one's intended use.

[normhead]

The advantage of a wider opinion aperture is more reliable AF. Pentax AF is optimized for ƒ 2.8 lenses.
Practically everything else posted here is nonsense. Factors like the age of design etc. what you consider sharp, ie edge to edge or centre sharpness makes a huge difference, more tha prime or zoom.

Check out numbers for the 50 1.8


Look at the numbers for the 16-85 at 50mm.


At ƒ8 and 50mm the DA 16-85 is 2502 LW/Ph and 2432 LW/PH on the extreme edge.

At ƒ8 and 50mm the DA 50 1.8 is 2415 LW/PH and 2341 LW/PH on the extreme edge.

I've also done comparisons that suggest the FA 50 macro is better edge to edge than the DFA* 1.4. There's no doubt the more corrected DFA* 50 1.4 creates much smoother out of focus areas, and in many case a nicer looking portrait, for one person. For group images I might go wth the 50 macro.

You can make whatever assumptions you want, but without actual lens testing, you don't know anything. I Have seen many instances especially with the older "Lenses for the way people take pictures, not for the test charts" lenses where the expensive lenses do not match up favourably with modern glass in edge sharpness (but often exceed in centre sharpness.) It all depends on what you're going for. They tend to be more centre sharp, less edge sharp.

But if you want centre sharpness and edge sharpness doesn't matter so much, for portraits etc. then the older, centre sharp lenses may be right up your alley. Not for group portraits but for solo portraits.

The issue is a lot more complicated than a few assumptions and generalizations. Different characteristics at different focal lengths need to be considered. And Prime is better than zoom can only be determined on case by case basis, as demonstrated above. In the case above, zoom is better than prime at ƒ8 (The other lens still might be better at ƒ5.6) . You can't assume, that just because you buy the prime for that focal length, you're getting a better image than you would with zoom.

You also have to consider the design philosophy of the lens, and how much your images might be affected by CA and distortion, and the execution of the design. The DFA* 50 1.4 is designed as a portrait lens, to be sharp wide open and produce excellent out of focus areas. As a landscape lens you might find it isn't quite as good compared to some other options, that were more designed for reduced excellence at smaller ƒ-stops, but to be sharper edge to edge. You have to know what you want to get what you want.

But to answer the main question, ƒ8 is ƒ8. However, the characteristics of a lens shot at ƒ8, in terms of sharpness, edge sharpness, CA and distortion, vary from lens to lens.

[othar]

Is a 'Fast' lens stopped down the same as a 'slow' lens at the same aperture?
I know we all lust after these F1.4 and 1.8 lenses, but I have a silly question I haven't been able to find the answer to:If you have two lenses, say an 50mm F1.4, and an 18-50mm kit lens with a F4.5 maximum aperture, and you set both at 50mm and F8. Is the same amount of light going to the sensor from each lens, or is the 'faster' lens allowing more light at F8 than the slow lens?
In other words, is F8 at a given focal length, the same sized hole, regardless of the maximum aperture of the lens? Or does the faster lens still let more light through, because it has a larger front element?Something I've been pondering, mainly because I have realized that shooting group portraits you can't get away with wide open apertures any way, so the value of the fast lenses seem over-rated to me.Frank

You've gotten many answers already. Please forgive me if for repeating some of them.

Beside the already mentioned T-stop topic the answer to your quesition is yes, both lenses set to f8 will result in the same image exposure wise (given you also keep shutter speed and ISO settings the same).
The smaller your aperture (or the higher your f-stop) the smaller the distinctions between different lenses will usually get.

The main differences will be noticed when the slower lens will be fully open while the faster lens is already stopped down a bit. -> for your kit lens this usually means beeing not sharp enough for large prints so I compare 2 of my lenses for arguments sake:
My Pentax-M Macro 1:4 50mm and my Pentax-A 1:1.7 50mm lens:
-My M lens is very sharp even wide open (not the sweetspot for sharpness but good nevertheless) so the main difference next to my A lens is bokeh: The Macro lens at f4 is fully open resulting in circular spots for background lights while the A lens already has hexagonal spots in the image because the aperture is stopped down at f4 (the shape of out of focus lights depend on the number of aperture blades and their form (rounded or not) -> bokeh is also influenced by how far your lens is stopped down: at f4 the aperture of my A lens is nearly hexagonal but before that it is a somewhat strange hexagon which results in a very busy bokeh)

[UncleVanya]

I've posted this a few times... And while it doesn't directly discuss the central question (which had been answered fully) it does addresses quality perception and reality particularly in shots that are not made wide open.

Nikon's 'Worst' and 'Best' Zoom Lenses Compared

[stevebrot]

The more I read this the more I find the terms "fast" and "slow" to be very strange ways to describe maximum aperture openings!

Tradition!


Steve

[normhead]

Tradition!


Steve

Some ƒ-stops lead to fast shutter speeds, some lead to slower shutter speeds. Same with fast and slow film.

If you want the fastest possible shutter speed, you buy the fastest possible lens. It's all about time of exposure.

[RGlasel]

I think this was the best answer:

I agree. Don't assume T-stops aren't important, in my own small lens collection I can find as much as half a stop of light difference between two lenses at the same focal length and aperture. If your purpose in using a faster lens is to get shallower depth of field, T-stops aren't important, but if you want faster shutter speeds with the same amount of light and aperture, T-stops can make a difference.

The important thing is to buy a lens that accomplishes the goals you have for photography.

That's the essense of buying lenses, period, full-stop. There are ways to accomplish subject isolation and smooth bokeh without using the fastest available aperture and there are many situations where versatility trumps barely visible differences in lens resolution.

[pentaxus]

It is hard to make a fast lens be as sharp as a slower lens. The 50mm 1.8 "plastic fantastic" is great because you can make a great lens with six lens elements. The 50mm 1.4 requires seven elements to get its sharpness. In old Nikon days, the 50mm f/2 was a sharper lens than the 58mm 1.4. Nowadays, a faster lens will have more expensive glass in it to make it better. Companies rarely bother to use the better glass in slower lenses.

[Kerrowdown]

Is a 'Fast' lens stopped down the same as a 'slow' lens at the same aperture?

Yes exactly the same in laymen terms, fast lenses are designed to serve a very different purpose... when shot wide open.

[mikesbike]

I know we all lust after these F1.4 and 1.8 lenses, but I have a silly question I haven't been able to find the answer to:

If you have two lenses, say an 50mm F1.4, and an 18-50mm kit lens with a F4.5 maximum aperture, and you set both at 50mm and F8. Is the same amount of light going to the sensor from each lens, or is the 'faster' lens allowing more light at F8 than the slow lens?

In other words, is F8 at a given focal length, the same sized hole, regardless of the maximum aperture of the lens? Or does the faster lens still let more light through, because it has a larger front element?

Something I've been pondering, mainly because I have realized that shooting group portraits you can't get away with wide open apertures any way, so the value of the fast lenses seem over-rated to me.

Frank

Short answer is yes, f/8 is f/8 on both lenses. Also, f/8 will not be wide open on either lens, and will be at or near the ideal for performance with either lens. And yes, if you do not need the benefit of having the aperture capabilities of the faster zoom lens, but also considering its being the more expensive, heavier, and likely the more limited FL range choice, then it is a waste of money. Some zoom lenses not having f/2.8 capability are nonetheless optically very good.

As to being over-rated, not at all. Again, it depends on the need. There are things an f/2.8 zoom lens can do that a slower one cannot. It can deliver a much faster shutter speed under the same lighting conditions for hand-held shots for low light scenes and/or for moving subjects. Such as: indoor sports events where such a lens can be indispensable. For example, my Sigma 17-50mm f/2.8 EX DC HSM lens can deliver 4 times the shutter speed at 50mm than your kit lens is capable of using the same ISO setting under the same lighting conditions. You can also open up such a lens lens to reduce your depth of field (DOF) to blur the background more to make your subject stand out better from it, and sometimes even get rid of a busy background in a scene. I have both styles covering a similar range, and it depends on the usage as to which I will be taking out. Again, if your shooting needs do not include such usages why spend the money for a fast lens?

[Frank29]

Thank you all for the ton of information on this subject. My question has been well answered!!

I fully understand the purpose of getting the best quality lenses you can, and that a fast lens gives you more options for shallow DOF and higher shutter speeds.

But, it also highlights how crazy this can get if you spend $1000 for the latest and greatest F1.4 prime when you already have a $150 F1.8 prime that is really really sharp. $1000 to gain 1 F-stop? I'd rather use the money on some other gear....