[lotech]

There are standard primes in every made like the commonly used 50/f1.4, but the size of the lenses can be quite big difference, for example the FA* 50/f1.4 is twice as big as the old FA version, but what made the difference ? I know that high refractive index glasses can bend light more so to make the lens shorter, and it is commonly used on zoom lenses with wide zoom range, but what about on fixed lenses, why all 'good' primes are so big in size ?

[Benz3ne]

I think it's due to the size of glass and number of elements required to correct things such as aberrations and distortion.
Personally, I don't mind a couple of quirks as a trade-off for portability, so I go for older, smaller lenses.

[kypfer]

It's all down to how "good" you want your lens to be
If you absolutely must have every last hair sharply in focus, right to the corners, at full aperture, with no aberrations, you're going to have to pay for the privilege … some of us are happy with a Tessar

[lotech]

It's all down to how "good" you want your lens to be
If you absolutely must have every last hair sharply in focus, right to the corners, at full aperture, with no aberrations, you're going to have to pay for the privilege … some of us are happy with a Tessar

Suppose we can produce a lens to prefect precision, by using high refractive index glasses is it possible to make the lens smaller that's what I want to know.

[Sandy Hancock]

The old FA 50/1.4 isn't remotely sharp until about f/2.8, and needs a couple more stops to get critically sharp. The new DFA* 50/1.4 is sharp across the frame from f/1.4

A lot of extra high-end glass goes into that difference. Add class-leading weather sealing and a powerful, silent in-lens AF motor and the extra size and weight shouldn't be too hard to understand.

[Jonathan Mac]

There's no reason at all that good lenses MUST be big. However, fast lenses that are sharp wide open are almost always big and heavy as they need a lot of extra glass elements to correct the aberrations that cause a loss of sharpness at wide apertures.

Digital photography has moved what most people regard as good image quality far over towards simply being sharp. Other elements are still there - contrast, bokeh, lack of aberrations - but sharpness is the most important one. I agree to a certain level but in most cases the size, weight and price of modern super-corrected lenses outweigh the benefits of the supposed improvements.

[lotech]

The old FA 50/1.4 isn't remotely sharp until about f/2.8, and needs a couple more stops to get critically sharp. The new DFA* 50/1.4 is sharp across the frame from f/1.4

A lot of extra high-end glass goes into that difference. Add class-leading weather sealing and a powerful, silent in-lens AF motor and the extra size and weight shouldn't be too hard to understand.

You probably right, it was bcoz the lens is too big and expensive too hard for me to try and 'understand' ha !

[yucatanPentax]

There's no room for an electronic motor in most older screw-drive lenses. That's one big difference.

Add weather-sealing which requires lips, surfaces, and seals in the body.

Then whatever glass differences are used today (which seem to all be aimed at the current trend in "test chart happiness" over anything else).

[BarneyL]

You can have a lens that is both exceptional optically and small, but then you have Leica prices, and probably no autofocus, since AF lenses are much harder to get right than manual ones.

[lotech]

There's no room for an electronic motor in most older screw-drive lenses. That's one big difference.

Add weather-sealing which requires lips, surfaces, and seals in the body.

Then whatever glass differences are used today (which seem to all be aimed at the current trend in "test chart happiness" over anything else).

Modern technology can produce small electronics components, like EVs can have motor built into the wheel did not add much to the dimension, the old FA lenses are similar size as the MF counterpart, so the most added to the size in the 'good lenses' are mainly glasses. Consumer grade lenses always need to stop down to produce good image where pro grade lenses can be used wide open, as far as I know on some design not the full area of the glass is used, effectively stopped down, that's one of the reasons those lenses are big in size.

[PJ1]

Suppose we can produce a lens to prefect precision, by using high refractive index glasses is it possible to make the lens smaller that's what I want to know.

To answer your question, no, such lenses cannot be made smaller. Lenses like the DFA* 50/1.4 are big because there is no other way to do it. To get the resolution it is capable of, with the f1.4 apeture, it uses 15 elements in 9 groups. And it weighs 910 gr. Compare that with another couple of Pentax 50/1.4 lenses: the FA 50/1.4 has 7 elements in 6 groups and weighs 220 gr; the Super Takumar 50/1.4 (8 element) has 8 elements in six groups and weighs 245 gr. If your lens absolutely must be sharp from f1.4, the DFA* 50/1.4 is your only option. If you are happy with 50/1.4 lens that is very sharp from f4 onwards (and maybe can beat the DFA 50/1.4 upwards of f5.6), then the Super-Takumar (8 element) will do the job. If you want a good compromise lens - more than sharp enough for most applications - the FA 50/1.4 will be fine. It depends on what you need (and as someone else said, a Tessar will do just fine - and cost about $30)

In addition, I would say that price is not always a good guide to capability. Here is an example. This is a shot taken hand held with the DAL 55-300 at 300mm, wide open at f5.8. The DAL 55-300 is an inexpensive kit lens weighing 425gr (I picked up mine second hand for $90 (AUD)). The first pic shows the original framimg. The second is the crop. I don't think I could have done better with a $2000 or $3000 lens with about 30 elements in whatever groups and weighing a couple of kilograms.

[Not a Number]

I think most people would agree that the Pentax "pancake" lenses are "good". These are not very large.

[Kunzite]

I think most people would agree that the Pentax "pancake" lenses are "good". These are not very large.

They are not very f/1.4 either

[UncleVanya]

The change in size is due to a change in philosophy of lens design particularly primes. Years ago Zeiss had a video interview where they articulated that primes were sometimes be lower quality than highly corrected zooms due to consumer tolerance for cost and size in primes vs zooms. Obviously the world has changed and very complex and larger primes are accepted by the buyers today. The cynical part of me suggests that the move is mainly marketing oriented and manufacturers are using this to lure us to buy lenses that we may not really need but we THINK will make our images noticeably different.

Note that in mirrorless where an EVF can apply software corrections there has been a move towards a hybrid lens design where electronic corrections plus optical design are married to allow some reduction in size with similar outcomes as large highly optically corrected lenses. Many will wrong their hands about this even mirrorless users but the output of these lenses tends to be very good in real use. Look at the Samyang Tiny series for Sony Full Frame to see how much can be achieved with good results. I’m not suggesting software corrections are free of impact - just that the aggregate should be compared so that we see the practical trade off.

I’m also not saying a dslr lens can’t use software corrections- we all know they can. With DSLR lenses the amount of software correction is typically a bit milder as the disconnect of what you see in the viewer vs the corrected image can get rather disorienting if geometric distortion is heavily corrected.

As photographers we have to decide when we really have a need for corner to corner sharpness wide open vs size. This is why the 18-135 vs 16-85 debate exists for example.

I personally use a lot of lenses mostly stopped down and this informs my choices. Even on my A7RIII I have an FE 28/2 that isn’t well loved by those shooting wide open. I own the FA 50/1.4 and DA* 55/1.4 and don’t see any reason to look at the DFA* 50/1.4 as my use of f1.4 is not massive. Would the DFA be better in head to head shooting… certainly! Would this difference be noticed without side by side comparison - perhaps sometimes. Would I carry the FA or DA* places that I wouldn’t carry the DFA? Yes.

[ChristianRock]

I think the CEO of Sigma really summarized it well a few years ago when he said "There is no magic in optics".

What he was saying was, if you want fast, highly corrected lenses, you need a lot of glass in them. What you get out of them, depends on what you put in them. Simple like that