Spellcaster with armor

[karaka]

I found an awesome piece of armor on D:15 that makes me want to switch from light armors with my DSFE:
+3 ring mail "Uxarriat" {rElec rPois rF++ rC+}
Apart from that, I found a +1 elven shield of protection, a staff of wizardry, and a ring of wizardry. I like the shield as well, as the high AC/SH overall gives makes engaging tough melee monsters and casting (range 1) sticky flame very easy.
When I equip all of this stuff, my main attack spells (sticky flame, mystic blast) work fine ("very good"), while my support spells where the relevant skills are not as highly trained (blink, regen, swift) are "poor" or "fair".
1. How do the mechanics of armor/shield/wizardry work with regards to miscasts/spell success rates?
2. To what level should i train armor and shield to negate any negative effects? I can not switch to Ash as I fear my current god's wrath (Sif Muna - last time I abandoned him, I kept getting miscasts for 50000 turns, which made a conjurer type character almost unplayable and left him with a lot of mutations).
3. Furthermore, I observed that my ring of wiz did not alter my spell success chances at all; does it not stack with staff of wizardry?

[lijacote]

When I equip all of this stuff, my main attack spells (sticky flame, mystic blast) work fine ("very good")

Oh man, no. Don't you know? "Very Good" is Crawl for "will definitely fail you at your weakest moment".

[Stormfox]

I don't have hard numbers at hand, but just from observation, play experience, and some memory of what the knowledge bots have to say, here goes.

Armour inflicts a casting penalty proportional to its effective EV penalty. Ring mail has a base EV penalty of -2, so it only takes 3 in Armour skill to get that down to an effective -1 and thus equal to leather. Shouldn't take you too much time and XP.

Shields, on the other hand, effectively reduce your INT. A regular shield is an effective -7 or so penalty to INT when it comes to calculating success rates. Elven gear reduces this penalty slightly. I'm not sure how the Shields skill mitigates this, but it does at high levels. If you're going to use a shield at all as a caster, use a buckler until you get a lot of Shields skill...but that's XP that you could have been spending on your magic.

"Very good" isn't all that good for a spell you rely upon. Certainly not if it's how you're killing things.

Wizardry sources do stack, but that shield penalty is huge -- enough so that a second source may not be enough to make a noticeable difference.

[karaka]

Finally lost him at Swamp:5 to an ice dragon due to extreme carelessness. In the interest of science, here is my morgue file.

I like high AC casters though. Any comments on my build appreciated.

  Code:

 Dungeon Crawl Stone Soup version 0.8.0-a0-5084-g27636db character file.

209032 Sucker the Destroyer (level 18, -4/96 (97) HPs)
             Began as a Demonspawn Fire Elementalist on Mar 9, 2011.
             Was the Champion of Sif Muna.
             Killed from afar by an ice dragon (25 damage)
             ... with a blast of cold
             
             ... on Level 5 of the Swamp on Mar 11, 2011.
             The game lasted 03:10:09 (64377 turns).

Sucker the Destroyer (Demonspawn Fire Elementalist)                                                        Turns: 64377, Time: 03:10:10

HP  -4/96 (97)   AC 27     Str 12      Exp: 18/326445 (2), need: 27554
MP  12/37        EV 17     Int 22      God: Sif Muna [******]
Gold 3454        SH 23     Dex 17      Spells: 13 memorised,  5 levels left

Res.Fire  : + + .   See Invis. : .   w - staff of wizardry
Res.Cold  : + . .   Warding    : +   x - +3 ring mail "Uxarriat" {rElec rPois rF++
Life Prot.: + . .   Conserve   : .   q - +1 elf shield {AC+3}
Res.Acid. : . . .   Res.Corr.  : .   p - +3 helmet of Nobility {Str+4 Dam+4}
Res.Poison: +       Clarity    : .   e - +0 cloak
Res.Elec. : +       Spirit.Shd : .   (gloves unavailable)
Sust.Abil.: . .     Stasis     : .   k - +0 pair of orc boots
Res.Mut.  : .       Ctrl.Telep.: .   M - brooch of Shielding {Ward AC+4 EV+4}
Res.Rott. : .       Levitation : .   f - ring of protection from magic
Saprovore : . . .   Ctrl.Flight: .   J - +5 ring of protection

@: repel missiles, regenerating, quick, very resistant to hostile enchantments, fairly stealthy
A: claws 3, powered by death 3, spiny 2
a: Channel Energy, Forget Spell, Renounce Religion


You were on level 5 of the Swamp.
You worshipped Sif Muna.
Sif Muna was exalted by your worship.
You were hungry.

You visited 8 branches of the dungeon, and saw 51 of its levels.
You visited 2 portal chambers: sewer, volcano.

You collected 4244 gold pieces.
You spent 810 gold pieces at shops.

Inventory:

Armour
 e - a +0 cloak (worn)
 k - a +0 pair of orcish boots (worn)
 p - the cursed +3 helmet of Nobility (worn) {Str+4 Dam+4}
   (You found it on level 19 of the Dungeon)   
   It affects your strength (+4).
   It affects your damage-dealing abilities (+4).
   It has a curse placed upon it.
 q - a +1 elven shield of protection (worn)
 x - the cursed +3 ring mail "Uxarriat" (worn) {rElec rPois rF++ rC+}
   (You found it on level 15 of the Dungeon)   
   It greatly protects you from fire.
   It protects you from cold.
   It insulates you from electricity.
   It protects you from poison.
   It has a curse placed upon it.
Magical devices
 a - a wand of hasting (3)
 o - a wand of draining (9)
 r - a wand of invisibility (2)
 v - a wand of polymorph other (0)
 y - a wand of teleportation (7)
 N - a wand of disintegration (1)
Comestibles
 g - 9 honeycombs
Scrolls
 b - 2 scrolls of identify
 i - a scroll of vulnerability
 m - a scroll of enchant weapon II
 n - 3 scrolls of fog
 t - 3 scrolls of magic mapping
 E - 3 scrolls of remove curse
 O - a scroll of teleportation
Jewellery
 c - an uncursed ring of sustain abilities
 f - a ring of protection from magic (right hand)
 h - an uncursed ring of protection from cold
 B - an uncursed ring of teleportation
 J - a +5 ring of protection (left hand)
 M - the brooch of Shielding (around neck) {Ward AC+4 EV+4}
   (You found it on level 6 of the Dungeon)   
   [amulet of warding]
   It affects your AC (+4).
   It affects your evasion (+4).
   Back in the good old days, every adventurer had one of these handy devices.
   That, and a pony.
Potions
 d - a potion of magic
 j - a potion of might
Magical staves
 w - a staff of wizardry (weapon)


 You had 2 experience left.

   Skills:
 + Level 1 Fighting
 - Level 1 Staves
 + Level 7 Armour
 - Level 9 Dodging
 - Level 4 Stealth
 + Level 9 Shields
 + Level 2 Traps & Doors
 + Level 14 Spellcasting
 + Level 16 Conjurations
 + Level 5 Charms
 + Level 1 Summonings
 + Level 6 Necromancy
 + Level 3 Translocations
 + Level 3 Transmutations
 + Level 14 Fire Magic
 + Level 7 Air Magic
 - Level 3 Poison Magic
 - Level 11 Invocations
 - Level 2 Evocations


You had 5 spell levels left.
You knew the following spells:

 Your Spells              Type           Power        Success   Level  Hunger
a - Flame Tongue          Fire/Conj      ######       Excellent   1    None
b - Repel Missiles        Air/Chrm       #######...   Excellent   2    None
c - Sticky Flame          Fire/Conj      #######..    Excellent   4    None
d - Orb of Destruction    Conj           #######...   Great       7    Honeycomb
e - Fireball              Fire/Conj      #######...   Excellent   6    Honeycomb
f - Fulsome Distillation  Trmt/Necr      N/A          Excellent   1    None
g - Swiftness             Air/Chrm       #######...   Excellent   2    None
h - Control Teleport      Chrm/Tloc      ######....   Very Good   4    None
i - Mephitic Cloud        Pois/Air/Conj  #######...   Excellent   3    None
j - Blink                 Tloc           N/A          Excellent   2    None
k - Regeneration          Chrm/Necr      ######....   Excellent   3    None
l - Iskenderun's Mystic   Conj           #######..    Excellent   4    None
m - Summon Butterflies    Summ           #####....    Great       1    None


[dolphin]

OT: So nagas really are THE armoured caster then? I knew you could get AC crazy with the barding and natural AC, but I didn't know they had a shield bonus also. Is that bonus from the size system?

[galehar]

Armour inflicts a casting penalty proportional to its effective EV penalty. Ring mail has a base EV penalty of -2, so it only takes 3 in Armour skill to get that down to an effective -1 and thus equal to leather. Shouldn't take you too much time and XP.

Shields, on the other hand, effectively reduce your INT. A regular shield is an effective -7 or so penalty to INT when it comes to calculating success rates. Elven gear reduces this penalty slightly. I'm not sure how the Shields skill mitigates this, but it does at high levels. If you're going to use a shield at all as a caster, use a buckler until you get a lot of Shields skill...but that's XP that you could have been spending on your magic.

This is completely wrong. Here are some charts for the progress of the evasion penalty with the armour and shield skills (spellcasting penalty is based on evasion penalty). You need armour at level 23 to bring the ring mail penalty lower than leather at level 0.
The shield penalty works mostly like the armour penalty, it's not equivalent to lowering your int.

Edit: fixed the link.

[dolphin]

Half base AC, but 9 AC naturally, and I think Armour Skill may affect this natural AC, too. Wearing just SwampDA (Skill 0, XL 27), the naga had 12 AC, 3 from the DA and 9 innate. At Skill 5, a naga gains 2 more AC, whereas a human gains only 1 AC.

Naga vs Human AC

(should this be in a different topic? sorry if I'm hijacking discussion)

[evilmike]

Wearing armour makes sense on almost all spellcasters, if you plan on doing any melee fighting at all. Casting is pretty easy in armours up to -3 ev penalty (dragon armour) and you get very good protection from these. You could consider something lighter if you're going for high level spells or if you are going to do all of your killing with spells, but for the most part it makes a lot of sense now to wear lighter/medium armours. You don't even need that much armour skill. Even on a naga or centaur (which get less AC from armour) you should consider it, because these races have poor EV anyway and you still get GDR from armour. For example, wearing dragon armour will guarantee that melee damage is reduced by about 1/3.

There are reasons for sticking with robes or leather though. Some I can think of are: if you're using ozocubu's armour (which doesn't work in heavier armour), if you're not doing any melee, or if you need the extra boost to cast level 9 spells (which can be a bit to cast without wizardry if you are wearing armour).

[Jk]

If you're looking for a heavily "armored" spellcaster, try a Transmuter of Chei. You end up with astronomically high AC/EV due to stats in the 30s while at full piety & 5 pieces of ponderous armour. Unfortunately, this build precludes the use of nice randarts (unless you want to take the hit to ponderousness, which is acceptable when you still have 4 other pieces). And some good transmuter races branch well into other spell schools.