I'm a bit confused about the focal length thingy of DSLR lenses. Can you clarify a bit?
I'm looking to buy Nikkor 40mm 2.8 for jewelry photography but everyone keeps saying that focal length is too less which makes minimum focus distance less too. So if I take jewelry pics and I'm not close to the minimum focus distance but a little further from the minimum focus distance, will it effect the pictures quality? I will be using the focus stacking technique.
Read the link
https://www.photo.net/discuss/threads/which-lense-is-best-for-up-close-pictures-such-as-of-jewelry.502055/
You would be better with a longer focal length, as suggested.
With jewelry, other factors come into play like reflections and lighting. If the camera is too close it may get in the way of lighting, create shadows and may even be seen reflected in the items.
With that lens if you move back the item will be smaller and that will have an impact on quality if the image needs cropping more.
The minimum focusing distance of that lens would be around one foot. I'm guessing that wouldn't be close enough for you. Get a macro lens instead, one that can focus to within inches. If you really want to be close, get a 60mm macro.
Buy a 60mm macro and use the depth of field focus technique. Save yourself a ton of work/post processing time.
Look into light tents.\:
https://digital-photography-school.com/how-to-use-a-light-tent-for-small-product-photography/
You need a true MACRO lens, … So obviously you are not going to take my previous advice and hire someone who has more knowledge and experience with this?
Others here, more experienced photogs, have mentioned two things, a 60mm macro lens and explained why; they've also mentioned controlled lighting, which you can easily obtain by using a light tent (or light box made from a CARDBOARD box). WHY do you doubt them?
Go to youtube.com and write in the search engine, photographing jewelry and/or jewelry photography lighting and/or jewelry photography using a light tent.
For jewellery and most small objects, a focal length around twice the "normal" focal length is good, because it enables you to get lighting into your subject without being obscured by the camera or your hands. Thus a 60mm macro for crop-frame (DX) is good.
Worry less about the focussing distance than about practicalities of lighting.